Combat shooting is a skill that gives you +11% critical skill chance per point up to a maximum of +99%, added to your own critical hit chance. Because the book gives you all 10 points into the skill at once this will essentially make all your shots critical hits.
The book can be found inside the Bastion during the later stages of the game, in the room with the large compass design on the floor. One of the bookshelves in that room will catch your eye, with high enough perception you will be able to find the book within a hidden compartment on the shelf.
This book can only be used once, so only one of your characters will be able to obtain this skill. Some people suggest using it on a squad member that uses heavy weapons or shotguns to get the most of this effect. Do not use it on your energy weapons character as energy weapons cannot critically hit. Personally I used this on my main sniper, I like having that one hit kill guarantee.