Minecraft wiki entry for Enchanted_Book, associated text: When combining items, the compatible enchantments from the book in the second slot are transferred to the item from the first slot, keeping the highest level of any type. If two enchantments have the same level and a higher level is available, they combine into the next level. If a book is applied to an item that can't take all of its spells, the appropriate spells are transferred, while the unusable ones are lost. Enchanted books are single-use.