A skill book recipe is a unique crafting ingredient found in the Big MT, and used in the creation of a skill book at a workbench. There are only thirteen such recipes in game, one corresponding to each character skill. The recipes are one-time use items consumed during crafting, preventing the creation of unlimited skill books.
Inside a footlocker found in one of the small craters. Fast travel to location, head forward through fence, third crater on left with the footlocker, after you fire the artillery cannon at X-7a "Left Field" Artillery Launch.
Hidden inside a vent in the Reception Observation Area of the X-13 stealth testing lab. Go across the center bridge (the one with the toolbox and footlocker at the end). When you reach them look to the left and you will see a rectangular vent which can be opened by jumping toward it and activating it in mid-air. The recipe is inside along with some random loot.