The threshold question: what is your definition of FP, are you a schemer, ML, haskell, or maybe scala or F#?

I could recommend a lot of things to read, (but you've probably already read them), by language designers Wadler, John Hughes, X. Leroy, Odersky, Bertrand Meyer, Rich Hickey. Also the Channel 9 videos of Meijer, Bright, and Alexandrescu talking about D and haskell were terrific.

And for books, you probably know about Pierce's TAPL, but Harper's PL book is rarely mentioned

http://www.cs.cmu.edu/~rwh/plbook/book.pdf

http://channel9.msdn.com/Blogs/Charles/Alexandrescu-Bright-M...

http://lambda.jimpryor.net/translating_between_OCaml_Scheme_...

and a debate about monads, since nobody ever debates about them

http://haskell.1045720.n5.nabble.com/Martin-Odersky-on-quot-...