Shared imported document missing target ‘Books’ app

Hello, I’m trying to import several documents into ‘Books’ on my iPAD from my MacBook Pro ‘shared’ via Airdrop. This usually works very well. Today however, after sending the document from the MBP the list of target apps shows on the iPAD, but the one I want (Books) isn’t there. Previously Books has appeared on the list, but not today. I cannot see why nor how I can restore Books into the target app list, there doesn’t seem to be a way to do that.

Can anyone suggest what I can do?