I will be bold:there are three outstanding books to which we can recur for a really deep question in Quantum Mechanics:Dirac's "Principles...", Kramers' "Quantum Mechanics" and Pauli's "General Principles of Quantum Mechanics". I will perhaps astonish you by saying that, of the three, Pauli's is the most authoritative. This matches the fact that Wolfgang Pauli was, for many decades, "the conscience of Physics", the oracle, we could say, were it not for the fact that he spoke a very direct language. One went to Pauli to know his reaction to one's "genial" idea. More often than not he demolished the thing. This happened to luminaries like Victor Weisskopf, Uhlenbeck and Gousmit and, yes, Heisenberg! It's a slim book. Actually it first appeared as a chapter of the famous Handbuch der Physik, in German, of course. The translation is fine, but a little IS lost of the author's style. It is an advanced book. You should read it after some good textbook, like Liboff or Griffiths. It will give you organization and perspective. But even if you read Landau's (and Lifshitz') "Quantum Mechanics", you would take profit of it. For you could compare two great masters in action. Pauli's treatment of Dirac equation is superb, and became the standard, including the notation. This is a classic. The very fact that it is still being sold gives me hope.