Minecraft wiki entry for Enchanting_mechanics, associated text: As enchantments offered depend on the enchantment level and the enchantment level depends on the number of active bookshelves, an easy way to change the enchantments offered is to disable bookshelves by placing torches between them and the enchanting table. That way one can still have the entire 'ring' of bookshelves around the table but get lower-level enchantments. Breaking the torches restores the effect of the bookshelves.