Handbook: House Davion is the third in the series of books detailing the nations of the Inner Sphere and is the definitive source book on the Federated Suns.
It includes a detailed history of the Suns, its capital New Avalon, and House Davion that begins in the 23rd century and ends in 3067.
It includes a description and brief history of many worlds that are representative of the Suns as a whole, and it lays bare the often byzantine, multi-layered bureaucracies that run the Suns government and the Armed Forces of the Federated Suns. It glosses over the military that Field Manual: Federated Suns detailed, spending more time on the society, culture, and economics of the most popular BattleTech faction, including brief descriptions of many of the companies that drive the Suns' economy.
Finally, the book includes a rules annex that expands on the sub-regions from Field Manual: Federated Suns and Classic BattleTech Companion, as well as rules for using the fax machines developed by the New Avalon Institute of Science, among other rules expansions and suggestions.
The book closes with new creatures native to the Federated Suns and new terrain and units for use in Classic BattleTech games.