I've just spoken with someone who has the same problem as this. Having used the exact phrase that I used to solve the quest, in the same case, he has failed to complete it.
Supposedly there are two quests, one where you're given a book and one where you're told to get the same book. I only had one of them, and it worked for me. The person I'm speaking to right now had both, and is unable to complete either. It appears to be a conflict.
And this is in the tech support forum. It's not asking for quest spoilers, it's asking for a solution to a quest bug.
A log used to go here. It was a link so that nobody could accidentally stumble into it, and it contained one tiny spoiler, one which is important in this issue, because it proved that there is a problem with getting the NPC to understand the correct phrase.