They are working for free writing a book that essentially boils down to helping give your app additional mindshare on the off chance that the book gives them a

They are working for free writing a book that essentially boils down to helping give your app additional mindshare on the off chance that the book gives them a decent return on their time. If there was a bunch of money in writing the book, you probably would have already written the book... and your book would be infinitely if not markedly better since you wrote the app/software. Nothing wrong with not wanting to document your free software to the point where the book is useless, but asking money from someone that wants to do this for you for free isn't something I would personally recommend.