I wonder why no one mentioned von Neumann's Mathematical Foundations of Quantum Mechanics. It is a little outdated and contains some minor errors but besides that it is one of the best books i have ever read. I would especially recommend it for people who study mathematics since it's written from this point of view and not in the sometimes strange physicists language (aka no bra-ket notation, no inexact use of mathematical facts etc). Since he is using (and for the first time formulating(!)) the theory of separable Hilbert spaces this is already accessible to anyone who knows linear algebra and even more so if you know some functional analysis. Most importantly von Neumann was a genius who could structure his thoughts in an extremely clear, highly logical fashion which makes it very enjoyable to read this book.