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)