I’ve done two little jams (I learned that shorter jams – but not too short – are the best idea). I am working on a public domain/open-source comic book character Jam. I even have a project page for it. It’s coming, I swear!
And there ARE book jams (like NaNoReMo), but usually they are for making interactive fiction or something like that.
Right now I’m in a Jam for adapting War of the Worlds to a game.