The only full-fledged, genuine quantum theory of gravity we have (and most likely, the only one that is possible for mathematical reasons) is string/M-theory so textbooks of string/M-theory represent the only canonical literature on quantum gravity that you may find as of 2011 and that is ready to be presented pedagogically to students as material they can further work with, see e.g. this list
Things that have been proposed as "alternative theories" can't really compete with string theory when it comes to the degree of rigor, strength of the connections with the previous established physics, and just simple pure internal consistency and the discouraging quality of canonical textbooks on these subjects is one of the simplest ways to see this fact.
In the same way, you can't really find any meaningful pre-1983 textbooks on quantum gravity, either, because this discipline wasn't understood, almost at all. Right before 1983, people working on the most similar kind of physics (except for 5 string theorists) would do research in supergravity which is really just a field theory generalizing Einstein's general relativity, adding extra fermionic fields and a fermionic symmetry (local supersymmetry) but it's a field theory. Most of the calculations they did were classical, just like in classical GR, and the attempted quantum calculations using the tools of quantum field theory were seen to lead to a divergent short-distance behavior. That changed in 1984 when superstring theory was shown to be free of anomalies and UV problems while it was capable of producing all the right classes of physical phenomena known from supergravity and gauge theories coupled to matter.
While the theories – quantum gravity and string theory – almost certainly have to be the same thing, the two names are used differently. "Quantum gravity" is reserved for the research of questions that can only be asked or that only become hard if the physical system respects both the postulates of quantum mechanics as well as those of general relativity (gravity). They're the questions of the type "how do the postulates or effects of quantum mechanics influence one or another situation where the curved geometry plays a key role?".
Some of these questions are answered by string/M-theory in its current state; some of these questions were approximately answered even by QFT tools before string theory; some of these questions remain open.
For example, the Wheeler-DeWitt equation (together with its various solutions such as the Hartle-Hawking state) mostly belongs to the third category (the things not yet established). It's the equation $H\Psi=0$, expressing the idea that the Hamiltonian constraint in GR actually encodes the full evolution in time, something that is possible due to the ambiguous meaning of the word "time" in diffeomorphism-symmetric theories. To solve it, one must first define his own time, by linking it to some coordinate-independent evolving quantity, and so on.
Partial arguments why this equation should be true exist, much like some approximate demonstrations how it could work in truncated schemes. However, at the end, this equation should only be applied to the Hilbert space of a full working theory of gravity. At this moment, and most likely not only at this moment, string/M-theory is the only theory that satisfies this condition. Unfortunately, the understanding of the Wheeler-DeWitt equation, if one exists, at the level of string theory is highly incomplete, to put it euphemistically. In fact, the equation itself is unnatural because the diffeomorphism symmetry is just one among infinitely many similar symmetries and the Hamiltonian linked to it is just one of many operators that should be treated on equal footing if they are treated at all. So it's questionable whether the Wheeler-DeWitt equation will ever tell us something new again or whether it has been superseded. Maybe, it should be replaced by some more complex structure we don't know.
Before 1983, the Wheeler-DeWitt equation was as confusing as today and our knowledge about it boils down to one or a few papers, most of which remain confusing. This has never been a stuff ready to be printed in textbooks and taught to students. It's a speculative suggestive work in progress that doesn't have to lead anywhere.
The Hawking (black hole) radiation is sometimes included into quantum gravity but Hawking's original calculation was done within effective quantum field theory, really ordinary non-gravitating quantum field theory on a curved background. So strictly speaking, it shouldn't really be considered a part of quantum gravity. In this way, he could have derived the black hole temperature. Indirectly via thermodynamics, this also implies that black holes should have an entropy and many microstates. Why they possess the required entropy had been a mystery through the mid 1990s when the entropy was microscopically computed in string theory – for the first black hole and then for dozens of others (lots of multi-parameter supersymmetric black holes, near-supersymmetric i.e. near-extremal black holes, and some completely non-supersymmetric black holes in which the stringy "tricks" may be applied as well). Aside from consistent and convergent formulae for graviton scattering amplitudes, this became a huge piece of new evidence that string theory is a consistent theory of quantum gravity and it remains the only theory that is able to solve either of these problems.
It's not true that all questions surrounding the information loss paradox have been resolved. While we know that the information isn't lost after all, the non-local processes that (as we know indirectly) surely take place in string theory are not well-understood. How far they operate? Why? How much can they change at all? Could they become observable in non-black-hole experiments? And so on. These questions remain mostly open.
A special part of quantum gravity is quantum cosmology. Here, we're not really talking about the common description of inflation that is needed to explain the cosmic microwave background; the latter is, once again, governed by quantum field theory on fixed curved backgrounds and shouldn't really be included in quantum gravity per se. Most of it remains inconclusive within string theory – even though people have already taken their fast interpretations what important processes happen when they talk about the multiverse etc. – and once again, it is not addressed by other approaches at all.
There are some other partial questions of quantum gravity that have been understood such as the changing effective dimensionality or topology of spacetime and so on (those things are allowed, do occur, and sometimes they are under complete calculational control). All these things have mostly been clarified by string theory. If you summarize the successes and failures of string theory as a tool to answer general questions about quantum gravity, the situations (including singularities) that are close enough to static ones (where supersymmetry may be preserved etc.) are well-understood in string theory; the heavily time-dependent situations such as the Schwarzschild singularity or the very initial point of the Big Bang are (mostly) not understood. But let me return to the original questions.
Prejudices vs insights
The word "prejudices" is clearly emotionally loaded. Such emotional labels don't belong to the realms of science that investigate totally plausible – and in fact, given the quantitative evidence, very likely – statements. I think that "insights" would be far more accurate but let's call them "general propositions" to be impartial.
String theory's general framework that quantum gravity should belong to is closer to the intuition of particle physicists who have worked with relativistic quantum field theories for decades before they were superseded by string theory. Even though string theory is no longer a local quantum field theory in spacetime, it still broadly agrees with some general propositions about quantum gravity, including
the dynamics has to be locally (at very short distances) invariant under the Lorentz or Poincaré groups, otherwise we would inevitably end up with contradictions with successful tests of special relativity even in the context of doable, long-distance experiments
the metric tensor field is another field in an effective field theory, a degree of freedom; it is fluctuating (in the quantum mechanical sense) which is why some unlikely effects (like information tunneling at the end, against the naive classical causality, see the last point) may be possible
the metric tensor has its particle-like excitations, the gravitons, whose existence (waves that solve Einstein's equations, the effective classical low-energy equations) and energy quantization (due to the periodicity of the wave function in time) may be deduced in a full analogy with the photons, independently of string theory
a related point is that the Hilbert space is naturally organized in terms of multi-particle states which may be created, in the long-distance limit, by Fourier-transformed linearized fields; perturbative expansions of the calculations should be possible (and are possible in string theory) and in the stringy context, they implicitly include linearized general relativity (which is just a tool to approach calculations, not any "violation" of any sacred principles, and one that is pretty much necessary to talk about particle scattering in any field-like theory at all)
the diffeomorphism symmetry acts on the spacetime coordinates but it's just a technical difference from the Yang-Mills gauge symmetries; in principle, both of them play the same role and in fact, they may be shown to unify in string theory or at least in some vacua (e.g. in the Kaluza-Klein way: but in a broader sense, string theory always unifies all particle species and all forces)
scattering amplitudes for the gravitons (and other particles) must be calculable (and the calculations must be anomaly-free, convergent etc.) because the scattering experiment may be done whenever the spacetime has a non-empty particle spectrum and an infinite, solvable asymptotic region (the flat space and the AdS space are typical examples)
local off-shell Green's functions that depend on $x^\mu_1, x^\mu_2$ etc. can't be defined in manifestly Lorentz-invariant descriptions of quantum gravity because the association of the coordinates $x^\mu_i$ with physical spacetime points is ambiguous due to the diffeomorphism symmetry; that's why these Green's functions wouldn't be gauge-invariant and why a consistent theory should only be able to produce the on-shell scattering amplitudes, unless we gauge-fix the gauge symmetry which is inevitably breaking the manifest Lorentz covariance (e.g. by using the light cone gauge)
the evolution should be unitary so the information shouldn't get lost even in the presence of evaporating black holes; the superficially inevitable proofs of information loss were shown to have loopholes and pretty much every professional physicist thinks that the conservation of the information has been shown to hold
Most of these propositions were really just guesses prevailing among particle physicists at the beginning (so they were never "codified" by textbooks) but have become pretty much indisputable (in some of them, I attempted to sketch the relevant proof in the very description) because of contemporary research and indeed, all known proposed "alternatives" to string/M-theory violate at least one of them or most of them.
The alternative attempts to construct a quantum theory of gravity generally start from the "classical general relativity" culture, taking the causal structure of spacetime completely seriously even though it should be a fluctuating quantum variable in any theory of quantum gravity. So these approaches want to "quantize" the classical starting point in a new way. A priori, this could look like an equally sensible and promising attempt. However, when one actually does the research, the symmetry evaporates: there doesn't exist any "reliable" or "systematic" way to obtain a consistent quantum theory from any classical theory; the relationship goes in the opposite direction only: quantum theories usually have classical limits.
In the particular case of gravity, all attempts to find a quantum version of the theory in a straightforward way immediately lead to highly discouraging results. One either runs into the usual problems of non-renormalizability when he tries to follow the methods of quantization from quantum field theory; or, when one tries to introduce some discreteness etc. by hand, one ends up with theories that violate the local Lorentz symmetry and most likely don't admit any "nearly flat" solutions at all. The infinite collection of undetermined non-renormalizable operators is just translated to completely equivalent pathologies and ambiguities in any discrete picture as well.
Aside from these general lethal problems, none of those theories could have ever addressed any of the serious "theory-independent" problems of quantum gravity such as the information loss paradox; the fate of the black hole singularities, Big Bang singularity, and the Big Crunch, and many others. Whenever one wants to address any of these issues, he has to add new arbitrary assumptions to the theory (the word "theory" therefore isn't really legitimate because this set of guesses doesn't really allow one to learn anything about anything). So the things that have been said and calculated about the alternative "theories" mostly remain a superficially quantitatively looking, but otherwise completely ill-defined, set of structures used as an excuse for the people who want to say that they're quantum gravity physicists but who don't want to learn string theory even though it covers a vast majority of the substantiated insights we know about quantum gravity today.