When using the Advanced Uncrafter:

If you remove the ingredients from an enchanted item BEFORE removing the enchanted book, the item is immediately uncrafted and you can't get the book.

Also, while uncrafting an enchanted item: Removing a book, then exiting the GUI, allows you to keep the book AND item. Repeating the process over and over will yield infinite supply of enchanted books.

*MC - Forge - Uncrafter versions in title