My top recommendation for someone like you (presumably with a statically-typed, OO, imperative background) is Scala. Scala is an imperative-functional hybrid with better support for OO than Java and most other languages around. Scala compiles to JVM bytecode (.net is also supported, though used much less) and provides Java interoperability unmatched among non-Java JVM languages. I use Scala in my compiler and have found writing Scala code exceptionally fun and satisfying. The best resource for learning Scala is Programming in Scala, written by the language designer himself.

If you don't want a hybrid language and instead prefer to jump straight into the fray of FP, I'd go with Haskell. Haskell is a purely functional language; there's no (first-class) notion of mutability or effect. My favorite Haskell resource is the witty Learn You a Haskell for Great Good!. In fact, I'd heavily recommend reading some of LYAH whether or not you intend to go with Haskell, since it's a fantastic introduction to the world of statically typed FP. I'd start reading it even before thinking about language choice.