Assassin's Creed Revelations Walkthrough Part 26 – Signs and Symbols Part II,

Head for the Column of Marcian, marked on your map by a Sofia Memory. Climb to the top of the pillar where you will find a marker. Accept the mission and switch on Eagle Sense. Look to the left for the one that is flat on the Aqueduct, under the broken section. Head straight for it, using the nearby building to reach where the book is hidden.

Head back to Sofia's to turn over the book and learn where to go.