This depends on the shelves you pick. Think about the types of books you want to store on the shelves (hardback versus paperback) and how many books you’ll place there. When in doubt, weigh the items you want to store on your shelves, and confirm that the floating shelves you want to buy are rated to safely support that weight. But, as a general rule, deeper floating shelves (without rod-mounted hardware) aren’t able to hold as much weight as shallow shelves.