I noticed this while updating (insert fork name) to 1.16.4.

In `PlayerConnection#a(String s, List<String> list, int i)`, the event is called as one expects. This method's name could be `bookSigningCallback`.

In `PlayerConnection#a(List<String> list, int i)`, the event is never called at all. This method's name could be `bookEditCallback`.

My solution to this issue was the following, though you may come up with something more future proof:

if (itemstack.getItem() == Items.WRITABLE_BOOK) {
  NBTTagList nbttaglist = new NBTTagList();

  list.stream().map(NBTTagString::a).forEach(nbttaglist::add);
  // Edits start - call PlayerEditBookEvent for non-signing
  ItemStack cloned = itemstack.cloneItemStack();
  cloned.getOrCreateTagAndSet("pages", (NBTBase) nbttaglist); // ItemStack#a(String, NBTBase)
  this.player.inventory.setItem(i, CraftEventFactory.handleEditBookEvent(player, i, itemstack, 
cloned));
  this.player.updateInventory(this.player.activeContainer);
  // Edits end
}

I apologise for not wanting to go through with the effort of compiling Spigot for this when the issue is that the behaviour doesn't exist in the code.