I've been pitched to write books a few times, by editors at reputable technical publishers. (HN readers would recognize the names of the publishers.) One of them told me up front that I would be offered an advance of $8K, that it would take 3-6 months to write the book, and that I shouldn't count on earning back the advance. She was clear that the benefits of authorship were reputational.

(The price wasn't why I turned down that and the other offers. Working on a book just isn't the thing I wake up eager to do. If I were an author I think I'd know by now…)