In Gene Wolfe's The Shadow of the Torturer, chapter 23, Dorcas appears – from the previous chapter one can figure out that she had been dead and had been buried in the lake. How did she awaken? In the second book the author makes it clear that it is not possible that dead people can be returned in their world. So it seems Dorcas hadn't been really dead. Is that explained somehow somewhere else?
Severian brings her back from the dead. He is an incarnation of the Conciliator, a Christ-like figure who has the power to heal the sick and raise the dead. Severian brings others back from the dead, also, including the dog Triskele.
Severian's powers will become more clear as you finish all 4 books. I am not sure what you mean by "author makes it clear that it is not possible that dead people can be returned in their world." Resurrection is an important theme of The Book of the New Sun.
In Citadel of the Autarch, when Severian is talking to Miles (the soldier he revived with the hooked Claw), he recounts what Dorcas said about the Claw when they saw each other for the final time in her room at the Duck's Nest in Thrax. She speculated the Claw had the power over time in much the same way that Father Inire's mirrors had over distance. She said, "When you brought the uhlan back to life it was because the Claw twisted time for him to the point at which he still lived." I would think that Dorcas, having undergone such "time twisting" herself, has a pretty compelling case. Only it's not the Claw, really, but Severian as we know from the aquastor Malrubius.
Because of Wolfe's religious views (staunchly Catholic), it's a great play on resurrection because modern Catholicism believes that the cause of ressurection is Christ as the Son (Sun) of God and not any artifacts while for most of the book, the cause of resurrection is attributed to the artifact -- the Claw -- and not until near the very end does it become clear that it's the New Sun (Son). (Having said that, there are clues everywhere.)