Also. for those of you who want to do your own campaigns, but find running through all these books a wee bit intimidating, someone has made MegaMek, a freeware BattleTech client (that features online play versus another play, or against a bot):
MegaMek | an unofficial, online version of the Battletech board game
and MekHQ (to handle the ins and out of running a unit)
MekHQ | MegaMek