Joe Armstrong (one of the erlang creators) wrote a book for Pragmatic Programmers. This is the book I used to learn erlang. It was quite an interesting ride, and I think it is an excellent book for learning the language.

I think erlang is a really fantastic language, and the only language that I personally trust for concurrency. It is battle tested, and once you spend a couple weeks learning the syntax, quite a delight to develop in.