Maybe a skull on both sides, real skulls, from the wizards used to create the book?
Each skull has it's own "spell list", and is treated like a separate caster?
To keep the party from being able to easily gang up on a single target to lock it down and gank it, maybe each skull has it's own familiar. But I'd make the familiars better than what can be gotten with the Find Familiar spell.
And maybe one or both skulls can cast a spell to summon aid?
Whoa, just realized that there's a person in the pic, and they're tiny compared to the book. If the book's supposed to be huge, maybe the skull(s) are those of giants from the first age? Hmm... as far as summoning aid, Storm Giants are CR 13, Hill Giants are CR 5, and the rest are between 7 & 9.