Field Manuals (FMs) are the bibles of the Army. They provide instruction and step-by-step procedures for just about anything and everything. There are well over 500 currently in use, with subjects ranging from infantry tactics, to knot tying, to nuclear operations...all the fun stuff.
There's even one with the title, War and Operations Other Than War (FM 100-15), which pretty much sums up the all-encompassing nature of Field Manuals.
Because of their general usefulness and utility, there is often crossover usage of FMs outside of the military in the civilian world. They are for the most part written in a straight-forward and easy-to-read manner, and usually have pictures and illustrations to go along with the text.
Some folks use them to figure out a sound way to cross a river, how to read a map, or how to overthrow a dictator and start a revolution while cooking a mean batch of biscuits and gravy. Others like to flip through various FMs, such as the one on First Aid for Soldiers (FM 21-11), and its Appendix B, which deals with aid-and-litter carrying techniques, and for no particular reason, recaption the illustrations to tell a crude love story…