The point isn't because of store abuse. It's because you'd be able to permanently have access to 50% bonus experience which isn't right for the gamemode.
By restricting the book, you only have access to this boost for 1 hour, as opposed to forever.
That's why I suggested making it so only the first book in a period of time gives bonus experience, after that you will be given a warning that the next book(s) will only award points until x amount of time.