Code Complete 2nd edition -- Every good programming practice and why to use it.

Structure and Interpretation of Computer Programs -- the book explains core computer science concepts, including abstraction, recursion, interpreters and metalinguistic abstraction, and teaches modular programming. Also what MIT used to use as their intro to CompSci textbook

Introduction to algorithms (Cormen, Leiserson, Rivest, Stein) -- Explains various algorithms and their benefits/drawbacks

The Mythical Man-Month -- What project management for software looks like when done right (and wrong)

Head-First Design Patterns -- Head First series is a little goofy, but you will remember how an Observer pattern works after going through their goofy examples. Gang of Four's design pattern book will put you to sleep.