The volume of books on open source tools is more a function of the number of such tools. For any given tool or language, there may be very few books in print.

Amazon has 12 books for "LLVM". [http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dst...]

There are over 1500 for "Visual Studio" [http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dst...]

343 books for "Django" [includes the musician] v. 1548 for "asp.net" [includes fibers woven for catching venomous vipers]

What does the volume of books have to do with anything? You seemed to indicate that you didn't think books were available for open source tools, but they do exist. What exactly are you looking for that isn't available?

One of "the benefits of Microsoft Press" and MSDN is the scale at which the documentation ecosystem operates. It's not a matter of finding a book, but rather of finding the right book.

I'm just trying to think of an open source tool where a comprehensive book (online or off) or some sort of documentation isn't available. Can you give an example of what kind of book you were looking for?

There is one definite book for Django, and it is very, very good. Why should be there any more?