Minecraft wiki entry for Enchanting, associated text: Combining two enchanted items, books or one of each with the same enchantment at the same level produces an item or book with the next higher level of that enchantment up to the maximum allowed in survival mode; for example, a book with Thorns I and Unbreaking II combined with a book with Unbreaking II produces a book with Thorns I and Unbreaking III.