As an author of a previous mobile/embedded techniocal book I can probably
shed light on this. It's just not worth it from an author perspective to
write a book in this space. A book is a _large_ amount of effort, and the
rewards are very small (I made about $3500 total off my eVB book, which took
a few months to write).
That's why, IMHO, you rarely see multiple books from the same author. The
rare exceptions are those authors who also do things like training, so the
books help show their expertise and relevence - it's not just a book, but a
marketing tool.
I was approached about a new book some time ago and declined the offer to do
one. I wouln't be surprised if other authors have done the same, and it
would go a long way to explaining why you don't see much out there on
bookshelves today. Our market is too small and publishers pay too little.
Until one of those change, or we come up with a different "format" for
content delivery, I don't expect the situation to change.
-Chris