Something with poetry - or word restrictions.
Like not using "e" - Or sentence length requirements - or maybe just overall word/character count limit.
Maybe a page limit.
Alternately - focus on the format of the book, create a template for a Booklet, or a folding brochure, then distribute that and the jam's goal is to work around that media/form.
Essentially, I think a creative limitation or restriction might induce some really creative work, and also makes a cohesive theme that is visual/distinctive.