About a millennium ago, sometime between 1020 and 1050, in Cairo, a large illustrated book on the heavens and the Earth was completed. Modern scholars were unaware of its existence until its recent ‘discovery’ and acquisition in 2002 by the Bodleian Library at the University of Oxford. It is today referred to as The Book of Curiosities, and it contains a remarkable series of early maps and astronomical diagrams, most of which are unparalleled in any Greek, Latin or Arabic material. The treatise is composed of two parts. The first is on the heavens, moving the reader from the outermost sphere of the stars through the spheres of the five planets visible to the naked eye down to the sub-lunar world of winds and comets. The second part is on the Earth, beginning with calculation of the Earth’s circumference, then moving to maps of the inhabited world, islands of the Indian Ocean and the Mediterranean, and major lakes and rivers of the world, ending with strange plants and animals inhabiting the Earth. Lost Maps of the Caliphs examines how the discovery of this manuscript contributes to the history of cartography, to the history of astronomy and astrology, and to our knowledge of Mediterranean and Indian Ocean networks of communication. It includes new perspectives on the history of maritime charts before the age of the portolans, on the patterns of Mediterranean travel and trade before the Crusades, and on Fatimid–Ismaʿili missionary networks in East Africa and the Indus Valley.