The book is aimed at experienced programmers of any procedural language like C#, Java or even C++ (but with possibly some exposure to Java and the JVM). This book is written by Martin Odersky, the creator of Scala, and it is very well written and packed with interesting examples that always match perfectly the topic that the author is trying to cover.
Because Scala is a fairly big language and because the author is sometimes a bit "chatty" (but always interesting), the book is fairly long. This is actually not a problem, because the first few chapters are actually enough to be able to start coding in Scala.
This is a great tutorial, but maybe, because of the way it's organized and written, not the best reference. However, the best reference (The Scala language specification) is freely available from the Scala website.
As for the language itself, I am very positively impressed with Scala and I think it is a big step forward from Java 6.