From the back of the book:

For nearly 300 years, war has raged across the Inner Sphere as the rival Successor States fought one another and fearsome invaders called the Clans. This maelstrom of conflict has created untold suffering and destruction -- and a never-ending demand for the services of mercenary soldiers.

BattleTech Field Manual: Mercenaries provides an in-depth look at mercenaries in the 31st century, from the inner workings of a mercenary unit to detailed profiles of more than forty prominent mercenary bands. This how-to manual for would-be soldiers of fortune includes the histories, officers and favorite tactics of select mercenary units. special rules that reflect the unique capabilities of each mercenary command, and rules for creating and running your own BattleTech mercenary units.

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This was the last update in the Mercenary's Handbook series by FASA; the sequel to the earlier Mercenary's Handbook 3055, it provides an update on a broad swathe of Mercenary units in the Inner Sphere, updates the rules on running a Mercenary band, earning contracts, logistics and the like, as well as adding some new equipment rules.

Several years later, FanPro updated this Field Manual to reflect current events in the canon timeline, and issued a new version as Field Manual: Mercenaries (Revised) under a new product number.