Marc Hedlund over at the O'Reilly Radar (yes from the same folks as O'Reilly press) has some good thoughts on beta books.
Why would I want to buy a technical book that hasn't gone through this sort of public review? For that matter, why would I want to buy a cookbook without the same system in place?
What are beta books?
Its basically a really low cost way of having a large number of people review and edit your book before you go to publishing. Basically you make it available online, either for free, or for a price. If you're selling the beta book, you probably want to also give the beta tests a copy of the book once its published.
The end result is that by offering your book to the public to edit online before hand you can produce a much higher quailty peice of work. And if your self publishing, you don't want to go to press with too many errata.