There's sort of an emerging Big 4 (plus 1) of FP<p><a href="http://book.realworldhaskell.org/" rel="nofollow">http://book.realworldhaskell.org/</a><p><a href="http://programming-scala.labs.oreilly.com/" rel="nofollow">http://programming-scala.labs.oreilly.com/</a><p><a href="http://caml.inria.fr/pub/docs/oreilly-book/" rel="nofollow">http://caml.inria.fr/pub/docs/oreilly-book/</a><p><a href="http://www.freetechbooks.com/introduction-to-objective-caml-t698.html" rel="nofollow">http://www.freetechbooks.com/introduction-to-objective-caml-...</a><p>don't know of any freely available content books on Clojure or Erlang;