Honestly, I want Koakuma to try to read the book, then have some sort of curse or abomination come out, then Patchouli saves the day, and Koakuma becomes eternally grateful and becomes nicer.
...Paint the book a different color? Hide it? Change the cover for something else so she doesn't find it anymore? Burn the book? Give it to Remilia? Give it to Flan? Tell Patchy that she has it, and will only deliver the book back if the library has been cleaned up again?
Anyone think the "Prank" could be Patchi leaving the book for Koa to read after saying it wasn't cursed, then when she gets to a certain point BAM Koa has been cursed anyway... maybe the curse known as << >> politeness. So evil that curse.
I predict that Koa will hide the book or do something to it, and Patchy will openly break into tears. Not the awkward , unsure upsettness we've had so far, I mean outright bawling. Koa will be totally unprepared for this, of course.