Book excerpt: In this innovative new history, Erin Pettigrew utilizes invisible forces and entities - esoteric knowledge and spirits - to show how these forms of knowledge and unseen forces have shaped social structures, religious norms, and political power in the Saharan West. Situating this ethnographic history in what became la Mauritanie under French colonial rule and, later the Islamic Republic of Mauritania, Pettigrew traces the changing roles of Muslim spiritual mediators and their Islamic esoteric sciences - known locally as l'ḥjāb - over the long-term history of the region. By exploring the impact of the immaterial in the material world and demonstrating the importance of Islamic esoteric sciences in Saharan societies, she illuminates peoples' enduring reliance upon these sciences in their daily lives and argues for a new approach to historical research that takes the immaterial seriously.