By Gene Wolfe. (This is the SFBC edition with all four volumes.)
I actually don't have much to say about this: it reads like a classic so it probably is. Severian, a torturer (torturers in Wolfe's world apply the punishments dictated by judges, of which long imprisonment is not one) tells the story of how he backs onto the throne. Others have said much more than I am willing to and probably in greater and more accurate detail.
Be warned, Severian has (and admits to having, in The Urth of the New Sun, which I did not read) a "weakness for women." He ends up having sex with practically every woman who crosses his path, which eventually (to me) becomes ridiculous. Your point of tolerance may be more or less than mine.
Interestingly, other critics apparently felt deceived as they read the end of the tetralogy. Apparently, I'm not smart enough to feel deceived, too smart, or was forewarned by reading such warnings when I was only perhaps two thirds of the way through.
Others have better reviews than mine: here is a good one.