I've been working on a book since March, 2012 called Akka Concurrency and on Monday, Oct 15th 2012, it went live at Artima. I'm really happy to see it make this milestone - it's been a long time coming!
The book builds on the existing, and excellent Akka Documentation by going beyond the what and the basic how of Akka and into the why, as well as covering some more of the esoteric bits of the how as well. It's a unique pleasure to see the wonderful documentation that the Akka team has developed. If they hadn't done such a great job, I believe that I wouldn't have wanted to write this book in the first place.
I hope that the Akka community finds it helpful and it brings more developers into this fantastic toolkit. Please head over to Artima and have a look at the book. Take a look at the table of contents, as well as the sample chapter and, if you like it, you just might want to grab a copy!