My bets are on it being a photo album with pictures of Remilia, Patchouli and Flandre being best buds.

Remember what Patchy said about alot of books being cursed? I think Sakuya put Koakuma in a bad situation.

3rd panel, is Koakuma crying or is she sweating?
Joking aside, I think the book is either Patchie's diary or a book written about Patchie's time with her friends (Koa included.)

It's probably something related to Patchys relation with the other characters, but I'd secretly LMAO if it was the Necronomicon or some other odd reference.