Last June, a pulp-fiction thriller was published in Paris under the title “Le Chemin de Damas.” Its lurid green-and-black cover featured a busty woman clutching a pistol, and its plot included the requisite car chases, explosions and sexual conquests. Unlike most paperbacks, though, this one attracted the attention of intelligence officers and diplomats on three continents. Set in the midst of Syria’s civil war, the book offered vivid character sketches of that country’s embattled ruler, Bashar al-Assad, and his brother Maher, along with several little-known lieutenants and allies. It detailed a botched coup attempt secretly supported by the American and...