Speak with Septimus outside Sanctuary and plot to break in. Head through the gate and use one of the headstones to jump to the wall. Move along till you reach the back of the building then use flail to shatter the window.
Once inside the head South then immediately East through the door. Let Septimus in and speak with him in the back of the room. After this head into the first-floor West room. The South room has two switches, hit them both and a pressure switch will drop down. Now leave the room and listen to Septimus, once finished head to the second-floor West room. In the South room you will find a hole in the floor, drop down to activate pressure switch to activate North door.
A vanishing book is in this room, catching it isn't possible alone, seek out Septimus for help. Chase the book around a bit and Septimus will the Book of Runes. Speak with Septimus again and he will tell you to find a key hidden in the Sanctuary.
Head to the second-floor East room and use the bow to shoot the lever behind the bushes. The South door will open, head in and look at the gap in the books on the shelf.
The shelf will open and another gap can be found. Keep this in mind and head back to the main room with Septimus. Listen again and then head to the first-floor East side door. Inside the North door the little door to the East is now open. Walk in to open a treasure chest containing a life vessel. Hit the switch and the South door will open. Upon entering all the seats will shift, replace the seats to their places and receive the Book of Verse.
Head back to the second-floor South door and place the Book of Verse into the gap. The bookshelf will shift and reveal a pressure switch, activate the switch to open the North door. Inside the room there is a lever.
Stay off the carpet, lever will disappear, by usings the chairs and table to get to the lever. Use them to get back and acquire the Curious Key
Now head back to give the curious key to Septimus in the main hall.
Upon entering the secret passageway Alundra will confront Ronan.