Quantum Mechanics Demystified - This is pitched at a higher level than 'popular science' books on the subject in that it assumes a knowledge of calculus and contains such equations throughout the book. Having said that, if you understand the principles of calculus, then this is a very approachable and readable book. Though be warned: the book contains a lot of typos and errors.

Feynman lectures on Physics 3 volume set.

A different universe: reinventing physics.