From the back of the book:

"COMPILATION OF six years of detailed research, The BattleTech Compendium may well be the ultimate resource for the player. Clear and concise rules clarifications, battle demonstrations, and rules changes will help develope and streamline play. All Material from the BattleTech Manual is presented in a revised form, as well as additional rules for JumpShip and DropShip combat 'Mech construction, and the latest discoveries concerning the technology of the Star League and the Invading Clans. Optional rules are included for miniature play, environmental effects on the battlefield, and more. Loaded with color photos, this Compendium gives the clearest picture yet of combat in the 31st Century."

User comment:

The Battletech Compendium is a compilation of all Battletech rules and rule clarifications. It is a concise complete guide including Aerotech, infantry and vehicle rules.

It also contains rules for 'Mech construction updatd to include inner sphere as well as clan technology..

The BattleTech Compendium superceded the BattleTech Manual: The Rules of Warfare and was in turn superceded by the BattleTech Compendium: The Rules of Warfare.