Running alongside the Mekong and Tonle Sap rivers, the Sisowath Quay is the main drag for tourists, expatriates, and international aid workers in the Cambodian capital of Phnom Penh. By day they flock to the grounds of the Royal Palace, with its famous Silver Pagoda and dollhouse-like Napoleon III Pavilion, and to Wat Botum, a golden spire–topped Buddhist monastery that in the 1930s fostered a young novice monk named Saloth Sar, better known later in life as Pol Pot. At night, Westerners push their way to the quay’s open-air bars and restaurants through a circus of street vendors: book…