As a newcomer to writing DSL's, I found the book very valuable as an introduction to all the intricacies that can be involved in writing a useful, nicely integrating, DSL. The abstraction of the 'semantic model', which Fowler advocates strongly, has proven indispensable, allowing me to easily build GUI-based DSL editor for an initially completely text-based DSL, without changing a single bit in the code executing the parsed DSL.

Incidentally, this book and the definitive ANTLR reference are exactly the two books that appeared on my radar when searching for literature in DSL's.