That said, I own a whole raft of Scala books, including the original by Odersky, the yellow-black Apress one and so forth; and of all of them, I find Wampler & Payne the most approachable and useful.

That's fair. Indeed, I've found that general purpose programming languages are not the ideal platforms for DSL development (my previous comment about Scala's fitness in this regard should be taken as a relative statement). That is, there are many idiosyncrasies of the host language that leak into the DSL, error handling/reporting is hard and tends to obscure the code, and the eventual DSL tends to have a very hosty flavor.

What we need are special-purpose languages designed for creating DSLs.