If you like Statically Typed Languages and [Brackets]:
Haskell while rather heavy on purity of programming paradigm, has one of the best introductions to a language I've yet seen in the form of "Learn You a Haskell For Great Good!". Book form is optional as the website is free but this is one author I didn't feel at all underwhelmed by. Real World Haskell is good but really is better taken as a follow up to [Learn..Good]. RWH is also free with optional papery goodness.
From Haskell you can transition easily to other ML variants like F# or OCaml both of which are more widely used. For the bleeding edge ATS.
If you like (and Dynamic_Typing Parentheses) :
Scheme is the more functionally oriented of the two main Lisp dialects, smaller and purer than Common Lisp but less widely used in industry. I personally like The Structure and Interpretation of Computer Programs but it's a fairly theory heavy book, that while a bit hard to swallow will make you a better programmer.
If you'd prefer Common Lisp either Practical Common Lisp or Land of Lisp seem like good investments, the first is free online and the second is fairly cheap and more importantly (for some) game focused. It's more powerful though less pure than Scheme.
You can also split the difference and learn both sort of at once with Write Yourself a Scheme in 48 Hours in Haskell.