After years as a Hollywood writer and filmmaker, Kristin Hahn felt a crisis of faith: she had no spiritual group she could call her own. Setting out on a three-year journey, she began an investigation of America's religious traditions, practices, and beliefs.

Crisscrossing the nation, Hahn spent a week cloistered in prayer with convent nuns and a month of Ramadan fasting with Muslims. She went door-to-door with young Mormon missionaries and head-to-head with turbaned Sikh yogis. She sat through marathon meditations with Buddhist masters and spent days in conversation and ceremony...