Shiv: Sharpening is a training manual found in the chapter "Bill's Town".
Overview[]
Obtaining this manual improves the durability of shivs that Joel can craft; instead of breaking on the first use, you can reuse it for an additional hit against human and infected enemies.
Shiv doors, however, will still break shivs upon being opened, and they must be opened with a full, unused shiv.
Location[]
After you meet Bill, take a look around his safehouse and you'll find this manual on top of the bar.