A Better Way to Die is a book revealed in Dishonored: The Corroded Man.
[Surviving fragment of an assassin's treatise, author unknown]
The true weapon of the enemy is their eyes, because with their eyes they can see where you are, they can see what you are doing, they can see how you can be defeated. You must use your own eyes first, and act so that the enemy cannot. The art of spying is as noble as the art of war itself.
The alchemy of war is a curious thing. If an enemy sleeps, they are already defeated, and if they wake later after victory is yours, they may never even have seen your hand in their defeat. The crafting of certain reagents to induce sleep is a necessary school to master, as is the development of immunity against such mixtures by the slow titration of self-administered doses.
To know your enemy is first to know yourself. To this end, every day must present a challenge, every moment an opportunity, to meet the person that you are. To search for your limits and to step beyond them. Only then can you be ready to face what may come, because only then will you know that of which you are capable.
All conflict is deception and lies, for to defeat the enemy, he must be fooled-fooled into thinking you far away when you are near, that you are sleeping when you are awake, that you are still when we are moving. And the greatest ally of deception is darkness, for it is only in darkness that we can truly see the path ahead.
The excerpts are respectively quoted at the beginning of Chapter 14, 18, the second interlude and Chapter 28.