Looks like we're getting a mix of Chamber of Secrets and Prisoner of Azkaban. Yay for ripple effects!
Let's see... I sort of want to give it to Touko, but she either might be more amused or more annoyed at a similarly cursed artifact meant to corrupt and kill. Besides, it's probably not best to put all the apples in one basket, so to say.
Sokaris I think would recognize the feeling of the book as emitted from Quirrel(?). Also, it would help us keep her close to the loop of one of the main central conflicts (the other being TATARI). Maybe she would discover something Touko couldn't/didn't, and vice versa, so they can compare notes. I at least trust the book in their care than through Snape/Dumbledore and ending up back at Hogwarts. AKA "the place that is never safe any year that Harry Potter attended."
And since Harry always wanted to meet Shinji's home, why not visit?
[X] Sokaris
[X] Fuyuki (Redacted)
(EDIT: [X] Kyoto)