when in Creative, a Shulker box containing items can be used to effectively duplicate items, such as 'original' books, by placing them and mining them again.

to recreate:
-enter creative mode
-place a shulker box and place test items in the box
-break the block - it will drop itself, containing the items as expected
-place the dropped block. the held item is not removed, so you can break and replace the box several times to duplicate the items inside...