Grimoires can be found in the occult idol. Be wary when taking it however, as it will lower your sanity and spawn a horde of spectres.

After three occult idols have been looted, the insomniac will have a chance to sell grimoires.

The grimoire is an in-game guidebook for Manic, depicting various features with cryptic drawings. Holding it unlocks early rituals as well.

As you progress through Manic, more grimoire chapters are unlocked by performing certain rituals. This mechanic is known as "insight" and also unlocks more rituals.

Insight level	Altar tier	Unlock recipe	Sanity charges
1		Obtain a grimoire
2	Deepslate	x2 Bone x2 Rotten Flesh
3	Deepslate	x2 Necroplasm x1 Lysosome x1 Moonstone
4	Anirrum	x2 Lysosome x1 Moonstone x1 Sunstone	3
5	Anirrum	x2 Lysosome x1 Elderskin x1 Oozing Nucleus*	3

* Note that the oozing nucleus is not consumed.