The note that Urag gives you in both of the "Fetch Me That Book!" quests (MGR20, MGR20B) isn't ever marked as a quest item. Since he will take the note back from you when returning the book (completing the quest), it's possible the developers didn't intent for it to be possible to drop or sell it while either of the quests is active. Not having the note marked as a quest item doesn't seem to have any potential of breaking anything though, like is case with a lot of quests.