From the back of the book:
"Beyond the Great Houses of the Inner Sphere lies the untamed Periphery. Filled with young souls seeking adventure and freedom, the Periphery is a watershed for every stripe of humanity. This diversity has spawned powerful Inner Sphere-like political coalitions, roving pirates, one-world lesser powers and bandit kingdoms striving for legitimacy. Wild, uncivilized and deadly, the Periphery is a turbulent frontier where any warrior who dares can be king.
BattleTech Field Manual: Periphery describes the various factions that make up the Periphery: Magistracy of Canopus, Taurian Concordat, Marian Hegemony, Outworlds Alliance, Circinus Federation, the Lesser Periphery States, Bandit Kingdoms and the Deep Periphery. This sourcebook describes the history, military doctrine, traditions and tactics of each Periphery faction, plus special rules that reflect the exclusive capabilities of each Periphery regiment, as well as new weapons and 'Mechs unique to the Periphery."