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. From fast traveling to the location, head forward through fence, third crater on left with the footlocker near the NW corner of the area, after firing the artillery cannon at X-7a "Left Field" Artillery Launch. You must fire the fixed gun located at X-7a "Left Field" first, however.
Hidden inside the SW 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 reached, look to the left to 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.