There's a lot of good books on the language, this is 1st edition of staircase book, which covers 2.7, so probably the majority was written for 2.8

http://www.artima.com/pins1ed/

There's also a book manuscript (downloadable from typesafe.com, and Manning.com has 3 books in the pipeline, tho the 3rd, on FP by Tony Morris doesn't show there.