Você está descrevendo " The Star Conquerors " de Ben Bova.
"Humanity is locked in a titanic struggle against galactic "bad guys" called the Saurians. However, the Saurians are under the control of a mysterious species called "the Masters." Humanity has never made direct contact with the Masters; all that is known is that it is a very old race of intelligent beings.
The narrator is Alan Bakerman, a humanoid that was once in the service of the Saurians, now an adjutant to the Frontier Coordinator and a military officer. His first assignment is to rescue the command of Geoffery (Jeff) Knowland, fighting a delaying battle on the planet Scandia"
- Charles Ashbacher Review