Which g factor in an EPR spectra is the right one? I was reading this introduction to EPR and it seems to imply both that there is a unique g-factor of a given compound, but also that we can calculate multiple g-factors for a compound.  For example, here's a picture on that page

This clearly shows more than one g-factor for mitoNEET & Fdx.  So is there only one g-factor or is it the g-factor at each specific value of the magnetic field that is unique?
P.S. Is there a good book/ primer you could recommend to learn more about EPR spectroscopy?
 A: The point of EPR is that you can learn about the B field in the neighborhood of an unpaired electron. You do this by applying a magnetic field, which generates an energy diffrence between the electron's spin up and spin down states. You measure this difference by finding the frequency of light that is absorbed. To a first approximation, $h\nu = g_e\beta B_e$, where $\beta$ is the Bohr magneton and $B_e$ is the applied field. (I am not sure of their notation.) So g is a constant of proportionality, independent of the applied field, at least to first approximation.
$g_e = h\nu /\beta B_e$
But there are other magnetic fields, the ones you want to measure. Primarily there is the field created by nuclear spin. There are other fields from nearby electrons that tell you about the chemical bonds. The effect of these fields is folded into g. 
$h\nu = g\beta (B_e + \Delta B)$
or 
$g = h\nu /\beta(B_e + \Delta B) = g_e + \Delta g$
g is sensitive to anything that influences B in the environment of the electron. largely this is the nucleus. To some degree it is the electron structure of the atom or molecule. So g is a property of the chemical. 
But if you solve for $\Delta g$, it sure looks like it would be affected by $B_e$
