From the back of the book:
"In a universe spanning more than a thousand light years, the mighty House leaders and Clan Khans rule their star empires with absolute power... or at least they'd like to believe so.
Illuminati. Genecaste. Order of Cincinnatus. Irian Corporate Cabal. Behind the House thrones and Clan Halls, a hundred or more such secret organizations wield immense power. Insinuating their influence into every level of society and government, from the Magistracy of Canopus to the Clan homeworlds, these hidden power brokers can make life difficult for the average Joe, or shake the very foundations of the Inner Sphere.
Interstellar Players describes the most powerful and influential people, organizations an entities behind the scenes of the Classic BattleTech universe. Some are firmly established, but shown here in a way you've never seen them before; others have only recently come to light. Gamemasters and players can decide which of these power brokers are real and which are paranoid fantasies. Any of these shadowy groups, from this sourcebook or the gamemaster's imagination, can become the ultimate villain in any type of BattleTech campaign. Let your imaginations run wild!"