The black sun rises
the eclipse of God's bright eye.
Glad angels. Selah.
The Book of the Old Sun is the answering antiphon to the Book of the New Sun, the Book of the Long Sun & The Book of the Short Sun. In it, the ties between Saint Anne & Saint Croix to Green & Blue are made explicit, & the history of the Dying Sun Earth (that's with a "Ea" & not a "U") is clarified. From the rise of North Korea as a super power & innovator in spaceflight to the terraforming & megascale engineering that takes the Moon to becoming Lune (& a suitable home for the inhumi/Annese) the Book of the Old Sun is a masterpiece of hard SF. The main protagonist is Inire (who will later become Father Inire, the Auturch's vizier) who is a half-abo from Saint Anne. Of course, the notion of whether it is the humans or the natives who survive on Saint Anne must be addressed, with the inhumi finally revealed as being the descendants of the protoculture that grew to prominence on Gondwanaland long before what we consider recorded history began. The rising drama consists of the turmoil & riots that the robotic servitors of the humans of Earth are throwing, culminating in the rebellion that sends the metal men to the stars. Inire's only rival is Pas Typoon, the middling bureaucrat with dreams of empire, who clearly is the the same emperor that launches The Whorl & confronts Severian in later books. All in all, a fitting companion volume.