Depends on you you can either "jump in the deep end" or you can gratually move more to it.
Gratually moving is good if you want to stay productive. (Scala, Common Lisp, dylan come to mind)
Jumping in the deepend will totaly go against everything you learned but you will learn very fast.
(Haskell, ML, Clojure and Scheme are probebly best here)
Books:
Land of Lisp/Practical Common Lisp for Commen Lisp
Not sure about Scala
Programming Dylan for Dylan
Learn you a Haskell for Haskell
Programming Clojure/Practical Clojure for Clojure
Structure and Interpretation of Computer Programmes/The little Schemer for Scheme
(Don't know a good book for ML)
I'd put ML closer to the gradual camp, though it depends strongly on where you learn it from, and partly on which flavor you choose. I see a whole lot of procedural-flavored Ocaml, and it's fairly easy (like Lisp) to mix procedural and functional styles.
Yes, the plan is to jump in the deep end, unwire my brain and apply the concepts I learn to my day to day Ruby/Javascript programming as much as possible. I think I'll go with Haskell.
Gratually moving is good if you want to stay productive. (Scala, Common Lisp, dylan come to mind)
Jumping in the deepend will totaly go against everything you learned but you will learn very fast. (Haskell, ML, Clojure and Scheme are probebly best here)
Books: Land of Lisp/Practical Common Lisp for Commen Lisp Not sure about Scala Programming Dylan for Dylan
Learn you a Haskell for Haskell Programming Clojure/Practical Clojure for Clojure Structure and Interpretation of Computer Programmes/The little Schemer for Scheme (Don't know a good book for ML)