This book focuses on aspects of Islamic thought in Iran and Yemen, and other regions of the Middle East, from the ninth to the fifteenth century CE, through a close study of manuscript materials. The book's seventeen chapters are arranged under five rubrics: Mu'tazilism, Zaydism in Iran and in Yemen, Twelver Shi'ism, Mysticism, and Bibliographical Traditions. The appearance of these studies together in a single volume makes this book a significant and welcome contribution to the field of classical Islamic Studies.