When combining like enchanted books the resulting item dissappears on click.

Example:

Anvil slot 1: Power 1 Enchanted Book

Anvil slot 2: Power 1 Enchanted Book

Clicking on Anvil Slot 3 for Power II Enchanted book results in item disappearing and leaves original books in slots 1 & 2.

The enchanted books in slots 1 & 2 after the resulting book disappears become inactive in the anvil and must be replaced to show new result in slot 3.