Repairing Items Completly enchanted and renamed with books isnt infinite

I assume this is a bug, I combined a brand new renamed diamond pickax with a SilkTouch Unbreaking Efficency 5 book, after repairing it 4 times it was to expensive. This defeats the point of using books to enchant your tools because it does not allow them to be infinitely repairable (even if its 30 lvls to repair would be fine ). If this is in the game then why have books when I could just enchant 2 diamond pic axes and combine them?