I love this idea, it's definitely something I would participate in.
One alternative approach could be to have the editor either provide an outline for the book, or simply write the first chapter and then accept submissions for every subsequent chapter. After each chapter is completed, the book up to that point goes live again for the other writers to read and they then get another opportunity to submit the following chapter. You then either receive a set bounty for every chapter that you submit that is accepted, or, as you suggested, you end up with a percentage share based on how much you contributed.
The reason I suggest doing it chapter by chapter is that it keeps the project linear and would save a lot of time and effort curating the content. It also allows the editor to have more control.
I'm just worried that if people were to be allowed to edit freely, small differences of opinion could cause books to never be finished. Working chapter by chapter means that as soon as the editor picks a chapter, that is done and the rest would have to be written from there.