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.
It's a sad thing if Wikipedians say it doesn't belong there as it is not encyclopedic and Wikibookians say it doesn't belong as it is not instructional material.
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.
WB:WIN - Wikibooks is not Wikipedia. The majority of Wikibookians, seeing that the book is deleted, see that a biography is not in itself an instructional resource. It may have fit in if it was part of a history of physics Wikibook, but by itself it seems that it wouldn't do (and even then, an "edit for time" would have been necesary).
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.
We do not allow any content that isn't considered to be instructional material. The spirit of WB:WIN is that if you want to expand a WP article, then do it on WP. Personally, I am opposed to any Wikipedia forking in general because of the fact that I consider encyclopedia articles are not by itself instructional material. Based on your post, [[The Biography of Nikola Tesla]] would have been deleted whether we have the changed policy or not, due to it originating from an edit war.
I have to say that, to some real extent, the "books" part of Wikibooks is really a misnomer: it's not clearly intuitive that we are writing textbooks of instructional material on this wiki.