The Castle of the Otter (by Gene Wolfe) is subtitled "a book about The Book of the New Sun". A report in Locus mistakenly said that the fourth book in The Book of the New Sun was to be titled The Castle of the Otter, and Wolfe liked the name enough to write a book to fit. In the book, Wolfe talks about how the tetrology was written, and why it has the form it does. He tells where the poems and all thoses weird words are from, and what they mean. He even has each of the main characters (well, all that were willing to) stand up and tell a joke. I found it delightful to read, for Wolfe has a wicked sense of humor. It is available from the Science Fiction Book Club. If you loved the tetrology , you'll probably enjoy this. If you haven't read the tetrology, go and get The Shadow of the Torturer (it's the first book).
Linda Wald