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.

User report:

This sequel to the earlier BattleTech: The Periphery (second edition) is the final volume in the Field Manuals series for BattleTech to be published by FASA. It was also the last expansion to be published by FASA, before the company ceased trading.

This Field Manual is closer to the ComStar Field Manual than the House-specific Field Manuals, in that it covers a variety of different nations and areas of interest. It was the first sourcebook to effectively treat the Marian Hegemony and Circinus Federation as states equal to the three long-established major Periphery states.

Whereas the other Field Manuals add new high-tech options to BattleTech, the Periphery Field Manual adds a low-tech option instead - the Rocket Launcher - a fairly primitive but dangerous set of one-shot weapons, and adds a fair number of assorted variants to existing 'Mechs that have been adjusted to carry Rocket Launchers as a part of their weapon arsenal.

This sourcebook was originally published by FASA in 2001, but was re-released with a new cover and new product code by FanPro in 2003. Both versions of this sourcebook are out of print, and it hasn't been released in an electronic format by the new license holder, That makes this one of the harder to find Field Manuals.