Glitch report, I guess, or maybe someone could clarify why it's not working? When I use a mending book on any tool or armour using an anvil, it comes out the same as it was, and doesn't apply the book. I haven't tested it for other books; only mending, though it may still be problematic. Here are some screenshots to help elucidate: 
In the screenshot, the pickaxe I put in has Unbreaking, Fortune, and Efficiency. The book I have applied is mending, and as you can see it doesn't do anything.
In the screenshot, the pickaxe I put in has Unbreaking, Fortune, and Efficiency. The book I have applied is mending, and as you can see it doesn't do anything.