My first introduction to functional programming was Land of Lisp by Conrad Barski. I found it nicely written, with a lot of cartoons (but not too many) to make it more engaging. I ended up liking Lisp better than the other functional languages, but then I haven't done any major Lisp projects yet.
I still recommend the book, though. It's a good (and yet fun) intro to functional programming, even if you plan to use a different language in the end.