Something I recommended a while back when the planar summons were changed. A cool addition to the summoning books would be planar books. I really like the randomization of it, but having holy (or unholy) books would be awesome.
I'd definitely make them rare (perhaps only obtainable in their respective planes?).
It would keep holy characters who want to be able to summon celestials and unholy characters who want to summon only demons from having to waste a spell - also - would generally be awesome and lead to all sorts of awesomeness all around.
Planar Books
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Planar Books
Owlbears are the only thing keeping Arelith from the D&D experience.