I placed a stack of around 40 books in my quick bar's first slot. I selected my enchanting table and hovered over the box and pressed 1 expecting a single book to be placed in the slot.
What actually ended up happening was the entire stack was moved but only one item remained in the enchanting table thus deleting 40 books.
This bug can be reproduced with any stackable item in any quantity and leave you with one item.
First place a stack of items in any slot of the quick bar
Second interact with an enchanting table to bring up the ui
Third hover over the enchanted item box and press the number key corresponding to the stack of items in your quick bar (i.e. first=1 last=9)
Hope this helps :] many cows died in vein tonight