I'm just starting graduate school in physics, and while I think many of the books mentioned so far are quite good (Greene and Feynman especially so), it's at least worth mentioning some of the more readable textbooks out there. The best way to get a feel for the concepts of a field like quantum mechanics is to actually get your hands dirty; the obvious drawback of this approach is that it takes time.
Another drawback is that textbooks tend to lose something in the readability category, although physicists are fortunate enough to have some fairly lucid undergraduate-level texts. David Griffiths in particular is quite a good author. His texts on Quantum Mechanics and Electrodynamics are reasonably in-depth with a focus on physical intuition over mathematical rigor, and are written in a manner that is accessible and (if you're a nerd like me) even entertaining. You should be able to get either at any decent library.
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Another drawback is that textbooks tend to lose something in the readability category, although physicists are fortunate enough to have some fairly lucid undergraduate-level texts. David Griffiths in particular is quite a good author. His texts on Quantum Mechanics and Electrodynamics are reasonably in-depth with a focus on physical intuition over mathematical rigor, and are written in a manner that is accessible and (if you're a nerd like me) even entertaining. You should be able to get either at any decent library.