Lying Awake by Mark Salzman. From Amazon: "Sister John's cloistered life of peace and prayer has been electrified by ever more frequent visions of God's radiance, leading her toward a deep religious ecstasy. Her life and writings have become examples of devotion. Yet her visions are accompanied by shattering headaches that compel Sister John to seek medical help. When her doctor tells her an illness may be responsible for her gift, Sister John faces a wrenching choice: to risk her intimate glimpses of the divine in favor of a cure, or to continue her visions with the knowledge that they might be false-and might even cost her her life."
