Reading Paul Dirac's "Principles of Quantum Mechanics" I have a similair question to the question here, but regarding a different book.
"Principles of Quantum Mechanics" is a 1930 work by British Nobel laureate Paul Dirac. The wikipedia article on this book states that there have been a couple of editions (with improvements and additions), the latest being the 4th edition, published in 1958 (6 years before the Feynman lectures were published). This is a long time ago and I'm wondering if parts of the book are outdated, and if so, which ones and why? 
 A: Dirac did the last revision of that book in 1967, in the chapter of QED.
The book is 100% valid, nothing is outdated. You may only notice some slight difference of style with more modern books, in the absence of drawings and sometimes in the notation (most integration indices are not explicit, for instance). There aren't any exercises as well, but nevertheless it is a very good book, well structured and very clear.
Together with Sakurai's Modern Quantum Mechanics, it is the best book for mastering QM directly in the Dirac kets and bras notation.
A: Dirac's book was once thought to be a very hard book to understand
(and because of its yellow cover, was called “the yellow terror”, 
an shared by Norbert Wiener’s book “The Theory of the Fourier Integral”)
but is actualy an extremely lucid account.
Its methodical treatment makes up for the lack of figures and pyrotechnics.
Be prepared to read it slowly - Dirac was famous for not wasting words - and rereading passages after reaching a deeper understanding.
A: I can't recommend anything written by Dirac as an introduction to the concepts in Quantum Physics.
The two most common textbooks for a beginner (which include exercises) are "Quantum Mechanics" by Alastair Rae. (most recent update 2008) and
"Introduction to Quantum Mechanics" by David Griffith (Most recent update 2004).
Both books cover near identical material, but have different examples and different approaches.
From there, a good continuation point would be "Introduction to Elementary Particles" also by David Griffith.
Finally, after all that and you still want more, I'd recommend An "Introduction to Quantum Field Theory" by Michael Peskin and Daniel Schroeder, this book covers the same material that Dirac's book, but is considered a lot more approachable. If you somehow get through all this, you know more Quantum Physics than most Physics graduates. 
As a matter of fact, Peskin and Schroeder is reading material for people taking their Theoretical Physics PhD!
A: The title of the book gives it away.
It's the Principles of QM. i.e the actual mathematical structure of the theory presented in a bare-bones, dry-as-dust format. It even states on the cover that it's a monograph, intended as a reference work for researchers and advanced students of the subject.
It's very thorough and as concise as possible so that reading it really requires a reasonable grasp of the subject to begin with. That is also why it's actually quite a slim volume despite the depth of cover. I don't remember seeing a single figure or illustration in the entire book.
Besides all that, although physics may not have come a long way since it was published, teaching techniques certainly have. IMO It would be a cruel trick to offer a textbook of that period to a modern student. 
