I might be just as happy co-authoring a book in one of the settings (when ladyperegrine and I were co-writing a book it was as fun as a good PBEM or more so) but I would definitely not be happier just writing one by myself. Writing solo is not nearly as fun as a PBEM. :) Writing solo does have the counterbalance of "maybe I can share this with more than just the players" which is a nice notion. Although with my disinterest in getting my work out there, it's not as big a motivator as it might be.

So, I dunno. If I had someone volunteering to co-author but not play, I'd consider that. I probably would not write either of these by myself, because I have lots of more-developed book ideas to play with already.