Clearly you botched that wuest. Start again and talk to the girl at the book store and the teachers.
Also keep lounging at the pool, looking into the principal office.
There is nothing to botch. There are no ways to restart the quest unless I restart the entire game.
Either the book vanished after one reading session (I can't recall, didn't notice until much later), or the item was overwritten in the inventory when I added another item to it (which is not that uncommon as a coding error).
Probably good to mention that I'm on my first run-through of the game, so it's possible this is a new issue that didn't exist in previous versions.
edit: restarted the entire game just to get an idea where it might have gone wrong. Apparently there is just one reading session that already progresses the quest, so it seems the most logical explanation would be that the book itself just vanished or was overwritten at some point from the inventory.