Everyone talks about Saudi money, but no one really knows what it is. Journalist Krithika Varagur, a longtime chronicler of religion and politics, tells the story of Saudi influence as it has never been told before, in a book reported across the breadth of the Muslim world, from Nigeria to Indonesia to Kosovo.

The Call connects the dots on Saudi Arabia's campaign to propagate its brand of ultraconservative Islam worldwide after it became oil-rich in the 20th century. Varagur visits diverse outposts of its influence, from a Saudi university in Jakarta to a beleaguered Shi'a movement in Nigeria. She finds that the campaign has had remarkably broad and sometimes uniform effects, from the intolerance of religious minorities to the rise of powerful Saudi-educated clerics. The kingdom has spent billions of dollars on its da'wa, or call to Islam, at many points with the direct support of the United States. But what have been the lasting effects of Saudi influence today? And what really happened to their campaign in the 21st century, after oil revenues slumped and after their activities became increasingly subject to international scrutiny? Drawing upon dozens of interviews, government records, and historical research, The Call lays out what we really talk about when we talk about Saudi money.