freeEbook_Scala
Functional programming meets object orientation
From Scala in Depth by Joshua Suereth

Implementing a REST endpoint with Spray
From Akka in Action by Raymond Roestenburg and Rob Bakker

What is functional programming
From Functional Programming in Scala by Paul Chiusano and Rúnar Bjarnason,

Consuming JSON
From Play for Scala by Peter Hilton, Erik Bakker and Francisco Canedo

Ad hoc polymorphism with type classes
From Scala in Action by Nilanjan Raychaudhuri

Scala: the New Web Architecture (PDF)