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.