Do you already have the standard undergraduate books?
* Introduction to Algorithms (Cormen, Leiserson, Rivest, Stein)
* Compilers: Principles, Techniques, and Tools (Aho, Lam, Sethi, Ullman)
Knuth is also great to have as a reference - I've only needed to consult it professionally two or three times, but it's saved a _lot_ of time when I have.