From the back of the book:
"For five long years the Inner Sphere has known war on a scale not seen in centuries, with the militaries of literally every faction involved in combat to one extent or another. Now, as relative peace appears to be settling over the Inner Sphere, a review of the military forces of the BattleTech universe are in order.
Classic BattleTech Field Manual: Updates updates all ten Field Manuals, including a history section that will bridge the gap between where the appropriate Field Manual left off and the current year of 3067, as well as a complete and current TO&E for all factions. Updated random 'Mech assignment tables for all factions are also included."