with your paperdoll open instead of droppd try
dropc #charid

That drops it back into the backpack, and isn't Exevent dropPD there specifically to drop it onto the PD? And why would it work fine with other spellbooks and not with one specific spellbook? Ahhh... Item type hmmm.....
But why would that matter if they're both classified as being "held" and I'm giving it the #findID value? Hmmm... but
are they both classified as being able to be "held"?
**** OK, I did some experimenting and found the problem: ****
If you pick up something (using Exevent Drag) and the item you pick up is
NOT an item that WILL drop onto the PD (such as a bank check), your script will be in trouble if it subsequently tries to use Exevent DropPD to drop it onto the PD.
Lesson learned: Exevent Drag in itself doesn't care whether or not it's "droppable" onto the PD - it's function is to simply pick the item up - which it does just fine. But if you then tell Exevent Drag to "drop" that "undroppable" item on the PD - it cannot - but it keeps trying, over and over and over and over - until you finally issue it an Exevent DropC #charID, whereupon it will dutifully drop the item into your backpack, just as it should. Isn't it interesting how these machines do EXACTLY what they're told? But then, that's exactly what they're designed to do, right?
Now I know what has caused me so much grief over the years, when I'll occasionally give Exevent Drag/DropPD to something that
should drop fine and doesn't.... So, if anyone is listening - could we somehow classify my Corgul's Handbook on Mysticism as dropPD(able)?