New main objective: Ask Urag about the insane book
Once again speak with Urag gro-Shub and ask him about the Ruminations on the Elder Scrolls, written by Septimus Signus (screen above), who has apparently lost his mind and therefore the contents is absolutely inexplicable. The mage will tell you that Septimus has hidden in some solitary spot and you will have to find him.
New main objective: Find Septimus Signus
Open the world map to establish that Septimus can be found north of the College of Winterhold. As the College is on a high cliff, you will first have to get down from it. Head to the city of Winterhold by crossing the bridge connecting the College with the main continent. Now the hard part comes, as you will have to find a spot from which you can safely slide down to the very bottom (screen above). You should find one east of the bridge leading to the College.
After reaching the water (screen above), you will just need to head north. Note, that you don't necessarily have to climb onto the island on which the College is, as you can also swim around it. The most important thing is to constantly move towards the point marked by the game.
Be careful, as on the way to Septimus you will come across, inter alia, Horkers. If you're having trouble eliminating them for any reason, you can luckily just run away as they shouldn't chase you. Your destination is a location marked as Septimus Signus's Outpost (screen above). Approach the small hatch and get inside.
Go forward and after reaching a small cave carefully go down and initiate a conversation with Septimus Signus (screen above). The crazy mage will tell you that the Elder Scroll should be at the Blackreach and ask you to record the contents of the Scroll inside a special dwarven artifact. He will also give you two items required to complete this mission, i.e. the Attunement Sphere, thanks to which you will reach the underground complex and the Blank Lexicon onto which you will transcript the contents of the Scroll.