Books qualify as a valid item for almost all enchantments in minecraft. Normally when a book is enchanted, the enchantments are stored to a different tag. This is to prevent the book from taking on the abilities of its enchantment. For example, you don't want people running around with fire aspect books setting things on fire.

To confirm this issue, place a book, or an enchanted book into the table, and give it an enchantment. The enchantment is placed on the active tag, instead of the stored tag. This also causes an effect duplication glitch to happen.