I've found Unknown Book, Vol. 1 and sold it to Urag, but the objective 'Bring Ancient Falmer Tome to Urag-gro Shub' is still active. How can I get rid of it?
If you play on PC I'd try the console command:
setstage DLC1VQFVBooks <stage> where
<stage> is either 0, 10, 15, 170, 200, 250 or 255. I cannot tell you which one, but I can tell you that 0 would set the quest as 'not interacted with yet' and 10 is the stage where you are asked to bring the book.
So 0 should remove the quest from the log but would re-enable you to do the quest. Therefore I recommend trying maybe even 100, which used to be the standard
quest completed stage.
EDIT: Setting the quest's stage to 200 worked for kotekzot. This has been done with the console command:
setstage DLC1VQFVBooks 200