I'll take this opportunity to once again plug Strang's excellent linear algebra series [1] with associated textbook [2]. Also right now I'm working through some Cornell lecture notes [3] (that read a lot like a textbook) on manifolds and differential forms. It's meant for college sophomores and juniors who have taken linear algebra and vector calculus, though it provides a very good calculus supplement in the appendix. For me, it's perfect: the tone is informal enough to provide motivation to continue, but the definitions and proofs are fairly rigorous. It's about the only self-study math text that I think I might finish.

Hope this helps, not sure if it's over/under your level.

[1]: http://ocw.mit.edu/courses/mathematics/18-06-linear-algebra-...

[2]: http://www.amazon.com/exec/obidos/ASIN/0980232716/ref=nosim/...

[3]: http://www.math.cornell.edu/~sjamaar/papers/manifold.pdf

Might be fun (depending on your temperament! but you asked...) to go through Principles of Mathematical Analysis by Walter Rudin (RIP). Also known as 'Baby Rudin.' (Try to forget that there's a 'Big Rudin'; it will only depress you.)

Baby Rudin is a tough book if you're not used to a spare definition-theorem-proof style. It's many people's first experience with real math. It demands a lot from you--when people say reading mathematics is entirely different from reading an ordinary book, they mean reading a book like this. But if you want superpowers, and you don't favor being bitten by radioactive spiders...

See this Amazon review: http://www.amazon.com/review/R23MC2PCAJYHCB/ref=cm_cr_dp_tit... Ignore the fact that he spells "chapter" as "chaptor", he's still on point.

How about "Concrete Mathematics: A Foundation for Computer Science", by Ronald Graham, Donald Knuth, and Oren Patashnik.

I have not finished it. Just starting out, but seems like an enlightening text.

I don't have a title for you, but I loved quantum computing course I took during my software engineering studies. Flexes your brain muscle in new and refreshing ways!