Aleksandar Prokopec, Learning Concurrent Programming in Scala. This is a detailed introduction to the basics of concurrent programming in Scala. In particular, it explores parallel collections and futures in much greater detail than this chapter.

Daniel Westheide's blog gives an excellent introduction to many Scala concepts, in particular:

For a discussion of cross-validation, see The Elements of Statistical Learning by Hastie, Tibshirani, and Friedman.