This document provides background information on Arthur Koestler, the author of the book "The Thirteenth Tribe". It then summarizes Koestler's book, which explores the history of the Khazar Empire, a Turkic kingdom that dominated a vast region between the Black and Caspian seas between the 7th-10th centuries. The Khazars unexpectedly converted to Judaism in the 8th century, and the book hypothesizes that many Eastern European Jews may be descended from the Khazars rather than the ancient Israelites. It discusses the rise and fall of the powerful Khazar kingdom, their defeat of Arab invaders, and their eventual migration into Eastern Europe after their empire dissolved.