I disagree with this to the point of saying that the Artima book is one of the most well written technical books I've ever read and the O'Reilly book is rather a mess. The Artima book is written by the creator of the language and while it is a couple of years old the information is still 95% relevant to the latest language changes.
I would recommend getting a copy of both of them, but you might want to check out the online version of the O'Reilly book before buying it:
http://programming-scala.labs.oreilly.com/
The Pragmatic Programmers book is not bad either, but it's only a short introduction to the language compared to the other two books mentioned.
All three books have nearly identical titles: Programming (in) Scala