How to Design (Declarative) Programming Languages

["Jenkins, Steven" <address@hidden>]

I have a friend who is struggling to develop a DSL, and I've been giving
him advice.  However, I'm getting a bit tired of basically teaching him
how to design a language, so I started to point him to Essentials of
Programming Languages.  

After reviewing the TOC, though, I'm not convinced it's the right book
to hand him.  Are there any recommendations as for a reference for
designing a language?  I'd leaning toward Watt's Programming Language
Syntax and Semantics, except that he's designing a declarative language,
so perhaps Watt's Concepts and Paradigms book would be a better choice?

Thanks,
Steven