MechWarrior 3rd also called Classic BattleTech RPG is the out of date 2d10 version of the RPG. Here is a small list of other books that support that out of date rule set.
*Classic BattleTech: Companion
*LosTech: The MechWarrior Equipment Guide
MechWarrior's Guide to Solaris VII
MechWarrior's Guide to the Clans
Field Manual: Mercenaries Revised
*These are probably the most useful to have. The Companion has a points buy system for character creation. LosTech is armor, weapons and gear.
Our group is playing the current rule set A Time of War which reverted to 2d6. However we converted it back to 2d10.