Lin Carter 
Star Rogue 

There was a name, but the name was a myth. The myth rode the tongues of ten billion men – and ten times ten billion not-men – on ten thousand worlds, and wherever the name was spoken, it was with awe and respect. Citadel!The secret name for the masters of space, the creatures who ruled without office, who served without reward.Citadel!The deepest mystery and the greatest power of all the myriad stars. Wherever two creatures met in fear of oppression, a silent cry to the men of that code-named organization rose in the birth of a new hope. For the galaxy was Citadel… and the Citadel was the galaxy!