Scala is one of the hottest new programming languages of the past few years. It's in use at companies like Foursquare and Twitter, and The Guardian is working on porting some of its Java code to Scala. Here are a few books and tutorials to get you started.
Programming in Scala
Programming in Scala by Martin Odersky (the creator of Scala), Lex Spoon and Bill Venners is the definitive guide to Scala. The first edition is available online for free, and the second edition can be purchased here.
Also, A Scala Tutorial for Java Programmers by Michel Schinz and Philipp
Haller is a 15 page PDF tutorial introducing Scala from the official Scala website. Scala by Example by Odersky is a 137 page PDF that goes into deeper detail than the short tutorial. It's marked as a "draft," so don't expect anything too polished.
Programming Scala by Alex Payne and Dean Wampler is part of the O'Reilly Media Open Feedback Publishing System. Drafts of books are released online in full to solicit feedback from the community. The print edition is now for sale.
Programming Scala is aimed at experienced programmers, but not necessarily Java programmers.
The busy Java developer's guide to Scala
A Scala Tutorial for Java Programmers is a 16 part tutorial by Ted Neward on the IBM developerWorks website. Neward starts by introducing functional programming concepts for developers used to objects oriented programming and works up to building a framework for a client library for accessing Twitter.
Scala for Java Refugees
Scala for Java Refugees by Daniel Spiewak is a six part series. Spiewak introduces the basics of Scala, then moves into using Scala for OOP.
A Video Tutorial on Scala
For those that would prefer a video tutorial, YouTube user ShadowofCatron has posted a series of Scala tutorials introducing the language, functional programming concepts, etc. Part one is embedded above.