I've already put up some discussion on this topic at the staff lounge:
http://en.wikibooks.org/wiki/Staff_Lounge#Forking_of_Wikipedia_content
This e-mail is mainly to advertise the larger issue to those who might read this list but don't frequent the Staff Lounge (I'm not sure exactly how many that is.)
The general gist of what I'm trying to propose is under what circumstances can content from a Wikipedia article be used as a seed to start an expanded Wikibook. A Wikibook about Nikola Tesla was started, in part, due to an edit war on Wikipedia where some Wikipedia editors wanted to add content, but it keeps getting cut due to the fact that the article already is quite large.
The Wikibook, titled "The Biography of Nikola Tesla", was IMHO a rather well put together Wikibook with title page, about 7 "chapters", and a couple of appendices. Indeed, other than the fact that it was the subject of an edit war at Wikipedia I thought it was a very well organized Wikibook, particular in comparison to most of the rest of the content on Wikibooks. It was a little sparse on content for Wikibooks, but that should be something that happens over time to expand and is precisely the purpose of Wikibooks. This Wikibook was ungraciously deleted over my objections because of a simple majority vote of those on the delete page.
I guess that I'm trying to modify the deletion policy somewhat to allow *some* forking of Wikipedia content, provided that the content on Wikibooks really is an expansion of the Wikipedia article and not just some POV fight or fork of Wikipedia content. The nature of Wikibooks certainly allows almost any article on Wikipedia to be turned into a book, provided there are interested parties willing to write the content. Forbidding any fork would, in effect, kill almost any Wikibook stub right now.