Well, there is The Magic of Houdini and Houdini on the Spot. Both of which are still useful, though the Magic book has sections that are somewhat obsolete (UI stuff in particular) but POPs, a lot of SOPs, CHOPs are all still the same. On the Spot has lots of tips that are still relevant as well.
Full disclosure: I participated in both
Andrew Lowell did a great e-book that he self-published that is also still very relevant. Google: Andrew Lowell houdini
To answer your question on why there aren't more books, that's easy: It's incredibly time consuming to do a book, and financially makes no sense. We're all super busy with work and/or families. Will Cunningham (author of Magic of Houdini) and I have talked about updating the book, and the publisher wants us to do it, but we both have kids and work. I'd love to do it, but I can't take 2 months off without pay from my workplace
Cheer,s
Peter B