is now
Ubuntu's sabdfl is looking for a way to write a book / books collaboratively over the web.
Being quite a name in the industry his comments section succeeds in attracting a lot of useful comments and suggestions. Not taking into account those cases where vim + svn are in fact the perfect match it looks like there is quite some voices in favour of applying a wiki-style front-end to a CMS to solve the collaborative editing of things.
That exactly has been the approach we've taken for our DaisyCMS, almost 3 years ago. And it has since proven to be working very well. (Except maybe for the wrong 'first appearance' this sometimes brings: hardcore CMS peeps think daisy is 'only' a wiki. And those in quest for a wiki, don't see use for (but only bloat in) those nifty CMS features.) Oh well, Daisy is a CMS, and you can use it as a wiki that will easily keep up with your growing needs...
Given the experience we got in building the "books" (as in 'paper' !) support into Daisy's (previous) version 1.4 we can also safely state that having the wiki+cms vision alone is not going to be enough. You'll need an editing environment that allows to introduce book specific metadata (some inline, some in a separate book-definition document to seperate concerns) and a shaped up publication mechanism that produces a paper-ready book out of a big cloud of reuseable wiki-entries.
Easy advise to those people struggling with the " Writing a book collaboratively" task: grab our book, and get onto our list.