The Citadel of the Autarch is a satisfying conclusion to Gene Wolfe's The Book of the New Sun. (A fifth book, The Urth of the New Sun, is a coda to the original four books.) We've known all along that Severian the torturer would be the autarch by the end of his story, but his fascinating journey to the throne is what this saga is all about... on the surface, at least.
What it's really about, for those who want to see it, is the juxtaposition of future and past, the nature of time and space, perception and reality, religion and science, and the Earth's and humanity's need for redemption. All of this is explored in the context of the strange characters, situations, and places that Severian meets on his way.
The Book of the New Sun is not an easy read, but it's what speculative fiction is all about -- it's brain-bending, it makes the reader consider and question, it stretches the intellect and opens the mind to new ideas and experiences. In The Book of the New Sun, Gene Wolfe accomplishes all this and does it in a beautiful way. This is my measuring rod for excellent fantasy literature.
For readers who don't want to be bothered by allegory and symbolism, or don't want to risk scorching their synapses, there's still much to admire in The Book of the New Sun, for though it wallows in weirdness, all of it is tied loosely together by Wolfe's lovely language, detailed world-building, smart ideas, and astounding imagination.
I look forward to reading on in Gene Wolfe's Solar Cycle (there are two sequel series: The Book of the Long Sun and The Book of the Short Sun.)