The First Stricture is a book found in Dishonored and The Knife of Dunwall.
Transcript
[Excerpt from a work detailing one of the Seven Strictures]
Restrict the Wandering Gaze that looks hither and yander for some flashing thing that easily catches a man's fancy in one moment, but brings calamity in the next. For the eyes are never tired of seeing, nor are they quick to spot illusion. A man whose gaze is corrupted is like a warped mirror that has traded beauty for ugliness and ugliness for beauty. Instead, fix your eyes to what is edifying and to what is pure, and then you will be able to recognize the profane monuments of the Outsider.
Location
In Dishonored, it can be found during the mission, High Overseer Campbell, in the Backyard. There is an office above the alarm, the book is on the table to the left of the unlocked safe. In The Knife of Dunwall it can be found in the Overseers' hideout in the Flooded District.