Generally speaking, books as a format make no sense. I have seen the financial records for number of popular tech book authors — and numbers related to them having published, for example: products, services, consulting & speaker fees, etc. If the author is trying, they make way more money from the numbers related to them publishing than the book sales. If you like writing long form tech guides, do yourself and your a…
I love buying pre-releas books from Prag Prog (which isn't exactly the same but similar to what you described). Especially since things change so fast, if you have to wait for a finalized, published version of the book, it's usually outdated by the time it even hits the shelves (not entirely of course, but parts of it). If the author(s) publish pre-releases frequently, it can be a fantastic reference.
That said, you would need to have a quantifiable definition of done and likely be a good idea to account for absolute failure in case something comes up an “done” is never accomplished. Until it is “done” likely be good idea to escrow the funds, but then you run into issue like paying taxes, transaction fees, etc.