The Book of the Wonders of Urth and Sky

The Book of the Wonders of Urth and Sky: Being a Collection from Printed Sources of Universal Secrets of Such Age That Their Meaning Has Become Obscured of Time, often called just "the brown book", is a book of fables and history from ancient days of Urth.

While in the Guild of Torturers, Severian is tasked with fetching this book, among some others, from Ultan's library and bring them to Thecla while she is imprisoned.

Some of these stories are told in their entirety in The Book of the New Sun and The Urth of the New Sun, others are published by Gene Wolfe as stand-alone stories, while others are just alluded to.

Stories originally contained in the brown book

 * "The Tale of the Student and his Son"
 * "Empires of Foliage and Flower"
 * "The Tale of the Boy Called Frog"
 * "The Boy Who Hooked the Sun"
 * "The Tale of the Town That Forgot Fauna"

Stories added by Severian
Severian adds some stories that he heard on his travels to his copy of the brown book. These are:
 * "The Two Sealers"
 * "The Cock, the Angel, and the Eagle"
 * "The Just Man"
 * "The Armiger's Daughter"

Fragments
Other references to and fragments from the Brown Book that are encountered in The Book of the New Sun:
 * A reading by Thecla from the Brown Book: "'It was the thought of Thalelaeus the Great that the democracy-that means the People-desired to be ruled by some power superior to itself, and of Yrierix the Sage that the commonality would never permit one differing from themselves to hold high office. Notwithstanding this, each is called 'The Perfect Master.'"
 * The tale of the angel struck by the child's arrow.
 * A dialogue between two mystes about the origin of culture.
 * The "three meanings" maxim (the thing the plowman sees, the soothsayers' meaning, and the transsubstantial meaning).
 * At least one story about the Conciliator, about which Severian is conspicuously tight-lipped.
 * Some fragments Severian sees while the wind is turning the pages to dry them: "soulless warrior!" ... "lucid yellow" ... "by noyade" ... "These times are the ancient times, when the world is ancient." ... "Hell has no limits, nor is circumscribed; for where we are is Hell, and where Hell is, there we must be." (Note: The last fragment is from Dr. Faustus by Christopher Marlowe. Noyade is a mode of execution by drowning.)
 * A picture of the sikinnis -- a dance of satyrs and sylphs
 * "Again she leaped, and twisting round the columns of the carapace..."
 * "Man, who is not wise, is yet the object of wisdom. If wisdom finds him a fit object, is it wise in him to make light of his folly?"
 * Pictures of angels swooping down upon Urth.
 * The tale of a man who drank the waters of another world and never returned
 * The tale of a great sanctuary where the face of the Increate is veiled behind a diamond curtain lest humans see His face and die

Possible other stories
Some short stories subsequently published by Wolfe but not explicitly referenced in The Book of the New Sun are thought by some to be from the Brown Book, or at least written in a similar fable-like style, but not confirmed to be from there. These include:
 * "The God and His Man"
 * "The Old Woman Whose Rolling Pin is the Sun"
 * "The Legend of Xi Cygnus"