I'd definitely second SICP if you've never done functional before. It teaches you functional concepts almost without you even realizing it. OTOH, if you are re

I'd definitely second SICP if you've never done functional before. It teaches you functional concepts almost without you even realizing it. OTOH, if you are really interested in being truly pure, it's hard to beat Haskell. Real World Haskell is a great intro for that language, and it's free online.