I'll keep it short. I've completed the main quest line and most of the side quests, found all of the Black Books, that sort of thing. I noted that I can't remove the books from my inventory, but that's not the problem. Somehow, Epistolary Acumen (one of the books from the main quest) has managed to disappear. I know I didn't put it somewhere because the game would not allow it. Any help would be great.

Are you sure when you finished the main quest you still had that one, you did not leave it on Storn's body? If you left it behind, go back to the Skaal village, Frea might have kept it for you, or the body might still be there.

Ah, thank you. Frea had it. It seems since I already did the main quest once and had to restart the DLC completely due to one of the Miraak glitches, I had forgotten to remove it from Storn's body.