Sciencefictionreaders.com

When Gene Wolfe finished the third volume of his The Book of the New Sun heroic fantasy epic, fans of the series began clamoring almost immediately for the fourth and final book in the tetralogy. As time passed and this concluding volume did not materialize, they began to root about for any information concerning the book, which did not even have a working title. The first glimmer was the title, which was announced by the Locus magazine as The Castle of the Otter.

The Citadel of the Autarch was the actual name of the final work in the series. Author Gene Wolfe liked the inaccurate title, though, and used it as the name for a companion work of non-fiction essays and un-used materials from the series (including an article about how Otter got its title).

The Castle of the Otter (1982, ISBN 0917488105) is something of a collector's item. It was briefly offered for sale through the Science Fiction Book club, while a limited edition vanity press hard-back version may have been published simultaneously. There does not seem to have been a mass market paperback edition of the work published in the United States since then.

The material in Castle of the Otter was combined with his short story collection Gene Wolfe's Book of Days to make the volume Castle of Days.