Technos by E C Tubb (1972)
Carrying a message for a dead man, Dumarest follows a new lead on the location of Earth that plunges him into a political coup attempt that ends up at a literal maze of death.
The Dumarest books generally deal with Earl Dumarest on harsh, relatively primitive worlds that, for all their technology and space travel, are mining camps or feudal societies. This time, Dumarest finds himself on Technos, an advanced, technological world where one's place in society is based solely on education and academic achievement. It's an urban planet where Dumarest's knife-fighting skills and woodcraft must be put aside for an ability to evade a sophisticated police force, handle a stolen ID and bluff one's way on a world where taking the train is a deadly hazard for a man on the run.
This is also a world holding a horrible secret. Dumarest smuggles himself to the close Technos by passing himself off as part of a human tribute from a vassal planet. At first, this is just a convenient way to gain entry, but Dumarest soon learns that there's a connection between the people sent to serve the rulers of Technos and the youthful appearance of at least one high-ranking lady–a connection that could doom a people to slavery and extinction. Meanwhile, Technos faces the choice between a coup of dictatorship by the increasingly insane head of the government.
And naturally, the Cyclan are there pulling the strings.
The seventh in the Dumarest Saga, Technos has the formula finally up and running. Dumarest is solidly established as a character, his world is well-defined, his quest understood and the Cyclan made a credible set of villains who can be defeated in each story, yet remain terrifyingly powerful. The urban setting in particular is a nice touch, as it shows how the formula is kept fresh by ringing subtle changes along the way.