The Empire of a Thousand Planets, Paperback/Pierre Christin

Valerian and Laureline are exploring Syrte, captial planet of a system of 1000 worlds. Their mission is to discover whether the Syrtians could present a danger to Earth. What they find is a decaying empire led by decadent aristocrats and a mysterious caste of masked wise men who discreetly pull the strings from hidden fortresses."