I've also been reading through all the documentation about the DLR as this environment is truly exciting for somebody with my kind of tastes!
Now - the documentation is quite, well, heavy; even if nice and detailed - and I'm hoping that with such a huge addition to the .Net framework proper (till now it's been a sideline) that somebody somewhere is preparing a book about it?
The theory behind expression-based languages is not really important to me (but might well be to other people) - but a good book that took you through implementing a language (a bit like the Sympl implementation in the docs), touching on all aspects of the DLR, would be really good.
Or is it the case that it's such a niche that there simply wouldn't be any money to be made out of it!? :D