- Erlang Programming: A Concurrent Approach to Software Development by Francesco Cesarini and Simon Thompson
- Programming Erlang: Software for a Concurrent World by Joe Armstrong.
I am currently in the process of learning Erlang for a personal project. These books both measures up to the high expectations I have come to expect from Pragmatic Programmers Publishing and from O'Reilly Books.
Erlang is a difficult language to "sell", and is a challenge to learn.
Both books assume you have decently good programming skills, and don't need your hand held too much about the idea of programming, and instead show you how Erlang is different, it's unique and interesting features, and some of "how to think in Erlang".
Both are very good books for learning the language, and gaining basic skill in using it.
Both of the cover pretty much the same territory, in pretty much the same order. You only really would need one of them, but they are both equally good, so I can't recommend one over the other. Either get both, or pick one at random.
I wish there was more on "how to think in Erlang", especially since most programmer's intuitions about multiprocessing and concurrency, born of battle scars with multithreaded programming in C/C++, will be wrong.