Do you want basics or advanced? At the moderately fancy level, Spray is great - any time you're using a directive, you can just click through in the IDE and see the implementation of what it actually does. And Doobie is a tour-de-force for advanced type constructs. At a more basic level a lot of Twitter's own libraries are straightforward readable code (e.g. Scrooge is one I've patched a bit myself), though coupled to the Twitter stack of Finagle/Scalding/etc. that you probably don't want to use.
After that you could look into the spray.io source: it is quite complex, but the API it exposes and how extensible it is is amazing... They also coined the _magnet pattern_ which is pretty cool, and probably less abused than the cake pattern.
http://www.squants.com/ seems like a great library for studying, since it's inherently intuitive, self contained, well documented, and exposes a terrific api.