References
By far, the best book on Scala is Programming in Scala by Martin Odersky, Lex Spoon, and Bill Venners. Besides being authoritative (Martin Odersky is the driving force behind Scala), this book is also approachable and readable.
Scala Puzzlers by Andrew Phillips and Nermin Šerifović provides a fun way to learn more advanced Scala.
Scala for Machine Learning by Patrick R. Nicholas provides examples of how to write machine learning algorithms with Scala.