Implementing Domain-Specific Languages with Xtext and Xtend is a book by Lorenzo Bettini introducing all the key concepts of language development with Xtext
DSL Engineering is a book by Markus Voelter that walks through all aspects of developing domain-specific modelling languages. Throughout, it shows examples in Xtext, but also using the MPS and Spoofax language workbenches. This may be particularly useful if you are interested in learning about the differences between these language workbenches.