I made an offer to Thomas Fuchs, but he was too busy working on the code
Even on relatively small libraries like P & S, there are several authors, and other kinds of input from various people too. Scriptaculous and Prototype are licensed under MIT-style licenses, and aren't owned by anyone, although Sam and Thomas are still undeniably the No. 1 Code Gods of P & S respectively.
Documentation is just one of those inputs. It's the nature of Open Source, different people bring in different skill sets to contribute.
Money isn't a major motivation for my writing, anyway. Sure, it's good pocket money, but not a major source of my income, and I wouldn't be doing it if I didn't think it was an efficient way to reach out to a lot of developers and help to build up the community.
I am benefitting from P & S by writing this book. Hopefully, I'm giving back too, by bringing new people to it, raising it's profile, and so on.
Dave