Definitely Programming in Scala - http://www.artima.com/shop/programming_in_scala. You might find some parts of the book a little boring if you already have experience with functional programming, but it is one of the best books about programming languages I've read (I've only read the second edition, not the one that is available online, but I guess, the first one will be OK as an introduction too).

"Programming in Scala" is a good one, I've been told. The first edition is available online at http://www.artima.com/pins1ed/ .

don't buy the wampler book as a 1st book. too many forward references, you are constantly flipping pages.

also, portions of the scala api are a work in progress. constructs for multidimensional arrays have changed several times.

i may recommend the wampler book when they come out with a rewrite, say in a few months time. Now it seems too rushed. Some of it reads like a collection of blogposts.

Also wampler's idea of functional programming is very basic. No details on commonly used functional data structures or functional pgmming algorithms. More like "functional pgmming is X as opposed to OO pgmming is Y".

Okasaki's book ( http://news.ycombinator.com/item?id=1138979 ) needs to be written in Scala. That'll sell a boatload of copies. Even a modest attempt at that would work for a start. That's an awesome book.

There is a PDF book called "Scala by example". It is a hands on approach that I find very interesting, because you learn by trying out applied cases.