it has been awhile since have done the main quest and am doing it once more before skyrim. i just got the fourth book from the sponsor in the sewer and it says i need to investigate farther into the books to see what i need to do next. i have read them b know quest notice pops up and i cant remember what to do. Thx

There's a secret message in the Commentaries, but if you can't figure it out on your own, you can take them to Tar-Meena in the Arcane University tower.