Scala Cookbook: http://shop.oreilly.com/product/0636920026914.do (most of it is available online here: http://scalacookbook.com/)

Once you are done with this, you can pretty much pick any of the projects here and start reading the source code:

https://github.com/lauris/awesome-scala https://github.com/adamw/awesome-scala

Reading code is the probably the best way to learn any programming language IMO