Just a quick update tonight: The issue of dual-licensed books is an old one, and one on which we've never come to any firm conclusions. There are some people who are definitely interested in writing some books under dual licenses, or alternate licensing schemes. Some authors, for instance, are interested in writing books which are completely PD, while others are interested in dual licensing with CC-BY-SA-x.x.
Wikibooks has, in the past, taken a very laissez faire attitude towards this. We have allowed books with these intentions to post message templates to that effect on the main page of the book. I have been very optimistic about this situation historically, although I am becoming more pessimistic as I get more experienced and more educated about licensing issues. I worry that an uneven licensing landscape will make content reuse between books more difficult, will raise the barrier to entry for new users, and will create problems for contributors who are not conscious of licensing peculiarities in individual books.
However, despite my pessimism, I do recognize that there is a lot of motivation on the other side of the line: Increased flexibility, better targetting of books to their target audiences, and an increased awareness of books as being individual projects in and of themselves. A book really is a closed ecosystem, with it's own authors, it's own styles, it's own guidelines and practices. Having it's own license seems like just another logical peice in the customization puzzle.
The question was raised again, first in the Reading Room (our equivalent of the Village Pump) and then on textbook-l where discussion is continuing. I would like to see a lot more feedback, from Wikibookians especially but also from other Wikimedians too.
I don't have time tonight, but I'll post some relevant discussion and textbook-l archive links tomorrow.