Since MechCommander was a tactical game where one commands multiple 'Mechs, i'd guess Mechcommander's Handbook has stuff for either larger scale games (unlikely, since that's pretty niche stuff and already covered in StratOps, and Alpha Strike does mid-scales reasonably), or perhaps some kind scenario-making book.
Unless it is some kind of fluff and sourcebook (like Field Manuals)? Some kind of rulebook seems more likely though.