Me.
It's not so much a sourcebook, but a missed opportunity in them. I am a 80s gamer who came up through the historical side playing you guessed it! Endless games in multiple systems of what happens when the Big Bad Russian Bear crossed into the Fulda Gap! (You gotta capitalize the Big and Bad and use the exclamation here!). I still have my copies of Isby's books about NATO and the Soviet Army.
So, where does this lead with Battletech? Well, to me, they missed the opportunities to really define doctrine for the IS armies? How do each of them approach what the Soviets would have called the Operational Art? (Yes, it's what they called it and when you look at what they did to the Wehrmacht in WWII? They were artists as time went on).
For example, a Steiner recon doctrine is going to be different than a Marik one, and a Davion different than either. The army capabilities are different, and the way they approach them are different. Trust, me, a US division commander and a Soviet division commander aren't going to approach taking an objective in the same manner. Hell, even definitions of the principles of military science are different. For example, the US thinks of surprise as a principle, and sees deception as part of that principle. The Soviets? They saw them as different principles, and planned accordingly, and built their army to match.
Sure, I doubt Battletech players are going to be running battalions on the table that often (other than using quick strike rules) but it adds something to understanding those armies besides "Oh, it's a Steiner Lance, guess no Hermes then". Furthermore, they could have done a rewrite of Battleforce that incorporated these "softer" factors (I thought the Command Chit was too, too random) and really captured the essence of the armies at that level.
Yes, I know...shut up Panzer, you are boring us silly.