I would suggest putting the focus not on the planes but on Summoning. Content could include professions (e.g. essence and channeling summoner professions), spell lists, details about summoning rituals, etc. Perhaps also a Banisher class, more spell lists for getting rid of stuff. Different reasons for summoning, not just to make them attack/defend but also calling creatures for divination, for analysis of items, etc, which will help make the professions more well-rounded. Then you've got room to discuss the wide variety of creatures that can be summoned, how they can be expected to act, etc. Cosmology provides an over-arching framework for the whole story, but because it is not the spotlight topic there is less need to provide firm answers. So, not Planar Law but Summoning Law.
Also, as a general principle, you sell more copies of books that have information required for character building than those that do not.