Artima's Programming in Scala is the best Scala book you can get at the moment. Very recommended. I found O'Reilly's Programming Scala to be a good second book; It moves a bit faster than I'd like though.
I also tried David Pollak's Beginning Scala but didn't quite like it, and I don't recommend it to anyone.
All the above-mentioned books are somewhat oudated as they don't cover a lot of things new in Scala 2.8. I am hoping that the updated editions of these books will be published soon.
Though more than half a dozen Scala books have already been published, I believe that the best books are yet to come.
The two books in particular that I think you should keep your eye on are:
Scala in Depth by Josh Suereth
Steps in Scala by Christos Loverdos and Apostolos Syropoulos