Ronald L. Graham, Donald E. Knuth, and Oren Patashnik. *Concrete Mathematics: A Foundation for Computer Science*. Addison-Wesley, 1989. QA39.2 G733X 1989.

A survey of mathematics likely to come up in advanced theory work with an emphasis on enumerative combinatorics. Much of this material is also scattered across KnuthSeries, but it's more accessible here.