Joe Armstrong's book is probably the best one to start with. It gives you a step-by-step learning process that will get you up and running with Erlang in short order. Once you are finished with it pick up Erlang Programming by Cesarini and Thompson; this book covers a lot of the same territory but goes a bit deeper into some areas that are missing from Armstrong's book (e.g. debugging & tracing, etc.)
The referenced book is THE book. It was an easy read for someone who had no functional programming experience. :)
I've been reading through the oreilly book, Erlang Programming. Its good (like all Oreilly titles) though I'm only into chapter 6. I'd definitely suggest picking it up.