The Art of War, by Sun Tzu. Or Sunzi, or however you prefer to spell his name. Either way, this work has got to be some kind of required reading. Indeed, it may be insultingly obvious. No insult intended, though. Beyond the sound military advice, Sun Tzu's analysis of the various qualities and failings of generals and rulers could be useful in cooking up some NPC's.
The Prince, by Nicky Machiavelli. Yes, again with the obvious. I want to make sure I've got the fundamentals down, y'know?
Fang of the Sun Dougram (太陽の牙ダグラム), directed by Ryosuke Takahashi. Yes, it's anime. No, I don't include it because it's where some of the Unseen come from. I include it because it's a serious story with high-level political shenanigans, decent military scenes where the makers put some actual thought into how giant robotic combat vehicles might work in battle, and a generally even mix of verisimilitude and dramatic license. The overall feel of this show is close to that of early days BattleTech. Hell, Crinn Cashim and Grayson Carlyle are cut from much the same cloth.
Armored Trooper Votoms (装甲騎兵ボトムズ), also directed by Ryosuke Takahashi. If Dougram is close to young BattleTech, then Votoms is BT turned up to 11. The war running for generations, the scavenger mindset, ex-soldiers drawn into fighting for gamblers, and probably a whole bunch more parallels that I just haven't got to see yet.
Beyond those, I don't know. Dirtside II from Ground Zero Games includes a brief bibliography of "Combat SF" on page 55. I expect some of those books (not enough of which I have read) ought to offer something to BattleTech. Here they are:
- John Dalmas - The Regiment
- Gordon Dickson - The Dorsai series
- David Drake - The Hammers Slammers series
- Roland Green - Peace Company
- Joe Haldeman - The Forever War
- Bobby Heinlein - Starship Troopers
- William Keith and Andrew Keith - The Fifth Foreign Legion series
- David Langford - War in 2080
- Keith Laumer - Bolo
- Ralph Peters - The War in 2020
- Jerry Pournelle - The Mercenary
Addendum: In conversation today Kevin suggested Armor by the late John Steakly for the list.