Definitely supposed to be itemstack.  itemstack is the intended new book item, as deserialised from the the Packet250 data. The newBookMeta for the event will extracted from that.  itemstack1 is the current book item in the player's hand; it's the source for the event's previousBookMeta.

This has been tested (and shown to work as expected) - see the testbookevents plugin linked in the PR description.