I spoke with Peter Bowmar at Siggraph about that. He said book publishers look at proliferation of programs before they decide to commision a book. They're having trouble convincing industry pros it's a must-have.
Also, he said pretty much every person qualified to write a book are working non-stop in the industry and don't want to take a pay cut to write a book.
but... books are over-rated. here's a link to all the videos from the sideFX site (over 1 gig of videos) that you can view to get a very decent head start:
http://www.vislab.usyd.edu.au/sidefx/houdi...ideo/index.html
http://sv1.3dbuzz.com/vbforum/uunr\_vtmlist.php?c=11
These are really great places to start. I should know. I'm just starting out too, but know maya.