Dirac's book is a perfect example of what I'm talking about. Just think about the fact that this book from 1958, which makes a valiant attempt at justifying a few parts of the formalism (it's on my bookshelf), is basically a high-water mark even though it's very flawed, especially given what we know today. What does it tell us about the ability of physicist to transmit ideas to the next generation with high-fidelity if, to understand why the quantum formalism is how it is, you recommend reading a 60 year old book by a guy who was alive when it was being formulated!
Some of those flaws:
(1) Dirac postulates, as most people do, that measurable observables are to be identified with Hermitian operators. This is a mistake that can be traced back to von Neumann. In reality, the larger class of normal operators are perfectly fine as measurable observables. Indeed, measurements are properly associated with only an orthonormal basis, and it is completely unnecessary to label them with eigenvalues, real or otherwise. (To see this, just observe that there is no physical difference between experiments that measure x and x^3 for the position of a particle.) Dirac's discussion on page 35 is just wrong.
(2) Unless my memory fails me, Dirac gives little to no justification for why we use tensor products to build up the state space of a many-body system from the state space of a single-body system, a tectonic shift from classical mechanics at the heart of the weirdness and power of quantum mechanics.
(3) Dirac was written before Bell's inequality. I mean, just look at the pithy discussion by Dirac (p. 4-7) to justify fundamental indeterminacy, one of the most profound things we know about the universe. Do you think this would have convinced Newton? Or Dirac in 1925? (We know it didn't convince Einstein.) This sort of thought experiment is lovely for an article in Scientific American to give laymen a sense of where things come from, but it's nowhere near the rigor with which we should teach physicists.