I will gladly put the opening section of the original Mercenary's Handbook up against nearly any other sourcebook if one wants to either impart or understand the 3025 era. Even with its mercenary-faction bias, it gives succinct breakdowns of all the major and some minor factions with a tightly controlled history back to the Star League, examines how forces are built, operations are conducted, and the types of missions and pitfalls a new player might face in-universe. The Kieths (J. Andrew in this case) were the acknowledged masters of the early 3025-era architecture, and I have always felt their standing was solidified with this work.
Further afield, it gives a decent system for unit creation, though that is not why I recommend it. The sample units are quite well-grounded in the 3rd Succession War timeframe, though, which is an added bonus.