"The Thirteenth Tribe: The Khazar Empire and Its Heritage" by Arthur Koestler is a historical nonfiction novel that delves into the story of the Khazar Empire. Published by Random House in 1976, this 255-page paperback explores the origins and cultural impact of this lesser-known historical period. With a clear narrative and in-depth research, Koestler provides readers with valuable insights into the legacy of the Khazar Empire and its impact on the world.