From the back of the book:
"THE STAR SYSTEMS on the outskirts of the Inner Sphere make up the area known as the Periphery. For hundreds of years, countless kingdoms and alliances have struggled against each other for survival in the far reaches of known space. Now, in the upheaval ravaging the Inner Sphere, these tiny realms may be the key to changing the balance of power among the Successor States.
In the wake of the Clan invasion a new era of exploration has begun. The intense efforts of the Inner Sphere bring the House Lords ever closer to discovering the Clan homeworlds. And every day of exploration reveals a more powerful, more complex and more exciting Periphery.
The Periphery sourcebook brings events in that area up to date, highlighting the larger and more important states in the Periphery and describing their current military forces, their histories, rulers, economies, goals and problems. This sourcebook for BattleTech also provides game information that includes 'Mechs, vehicles, personal gear and character archetypes unique to campaigns taking place in the Periphery."
While this sourcebook is largely thematic fluff, it does include 16 pages of rules, units and equipment to expand the basic BattleTech game.