When anviling an overleveled enchanted book, you will not get the enchantment of the book but the maximum enchantment level for the specific enchantment. E.g. anviling an unbreaking IV book will give you an unbreaking III enchantment. I could not find anything in server files to enable this. I thought of adding an anvil event to basically set the appropriate level when anviling, but is this really necessary? Isn't there a direct way to do this? The same thing goes with items. You cannot anvil an unbreaking III pickaxe with an unbreaking IV and get the unbreaking IV enchantment.