From the back of book:

Over 250 years ago, Aleksandr Kerensky, Protector-General of the Star League, left known space. With him he took almost half the MechWarriors, BattleMechs, and support personnel in the Inner Sphere.

For 250 years, the Successor States have battled for supremacy. Technology has eroded, with both manufacturing capabilities and knowledge lost on the battlefields.

Now the heirs to Kerensky's warrior spirit are returning, and they have suffered no loss of military technology. Instead, they have honed their weapons to a keener edge since their departure, and the objective of these warrior clans is nothing less than domination of the Inner Sphere!

The BTR: 3050 details all of the new, state-of-the-art BattleMechs, both the improved Inner Sphere 'Mechs and those used by Clans. Also included are construction rules for building new 'Mechs using the awesome new weapons, armor, and heat sinks that are available.