Skyrim Walkthrough: Hitting the Books

You obtain the quest "Hitting the Books" from the Arch-Mage, after completing the Under Saarthal quest.

Since Tolfdir is not available, you get asked by Mirabelle Ervine to look into information on the object we found in Saarthal by talking to Urag gro-Shub in the Acanaeum

You can enter the Arcanaeum through the Hall of the Elements. Talk to Urag behind the desk and tell him you want to learn about something you found in Saarthal. He will tell you he can't help since his books got stolen by Orthorn when he went to Fellglow Keep with the others who left the college. Apparently, one of the volumes Orthorn stole has the information you need.

If you look at the quest journal, it has been updated: "I've been asked to find more information on the object found in Saarthal. Urag suggested that useful books may have been stolen from the Arcanaeum by Othorn, a former apprentice at the college. His last known location was Fellglow Keep." You will also have something that shows how many of the three stolen books you have recovered.

NOTE: I would advise bringing Lydia with you if you are not above level 10 when starting this quest as there are a number of difficult opponents. It would also benefit you to have a ward spell since you will be fighting mostly mages during this quest.

Ancano will ask you some questions as you leave. Answer his questions and leave the college. Head to Whiterun, leave via the main gate, and travel East to Fellglow Keep (it's past the Honningbrew Meadery and White River Watch).

NOTE: It would be a good idea to pay attention to the books that are scattered throughout Fellglow Keep (not just the ones you need) because there are many spelltomes as well as books that increase skills.

Apparently there is a main door which is locked and requires a key, but you can find a door in a ruined tower which you can get in by. I believe this door is the one that is marked with a quest marker when you have this quest active.

Work your way through the flooded caverns. Go east, then south, kill a mage and 2 spiders, then head NE and arrive at a room with some cells. Activate the levers to release the imprisoned vampires, then keep going NE to another chamber with jail cells where a mage will release wolves at you. Once you have killed the mage and wolves, talk to Orthon through the bars and tell him you are here for the books.

He will tell you that if you free him, he will help you find the books. Free Orthorn by activating the middle lever. Open the wooden door and go down the stairs. Orthon will help you fight a group of mages after which you need to climb up the stairs, go to the end of the corridor and turn left. Open the door to Fellglow Keep and head NE towards the Fellglow Keep Ritual Chamber. There is basically only one path you can take which will bring you through multiple rooms with a number or opponents. Make sure that before you get to the Ritual Chamber, you kill the mage who has the Fellglow Keep Key (should be the last person you kill before getting to the door to the Ritual Chamber). Go through the wooden door to the north and climb up the stairs to the Ritual Chamber. Be ready to fight when you enter because the Caller inside is quite a powerful mage for this stage of the game.

You will find the Caller inside this room. At some point in the conversation you have no choice but to tell her you are not leaving without the books. Fight her, kill her and take the three stolen books (these all should have quest markers to show you where they are). Loot the caller for the Ritual chamber key and enter a small room nearby with lots of treasures. Then exit down through the grate on the floor. Raise the bar and open the wooden door. Use another door to exit to Skyrim. Fast travel to the College of Winterhold and go the the Arcanaeum to return the books to Urag.

This complete the quest "Hitting the Books" and unlocks the next quest in the College of Winterhold series, "Good Intentions"