Visual Complex Analysis - Needham. A lot of great pictures, exerything is explained (and shown) geometrically. Complex differentiation/integration, non-Euclidean geometry, vector fields etc.

Mathematics and its History - Stillwell. Wonderfully written. From the ancients to the 20th century, covers the mathematics (in a lot of detail) and lives of major mathematicians (potted bios).

Concrete Mathematics - Knuth et al. Discrete maths, number theory, generating functions, covers most of the maths that pops up continually in programming.

All You Wanted To Know About Mathematics But Were Afraid To Ask. Mathematics for Science Students 2 Vols. - Louis Lyons. Pretty basic, but covers all the mathematical basics for physics. (1st year uni level I guess) I learned a lot from this years ago, I still consult it as a reference. It's a pleasure to read.

Also, all of Hamming's books.