Mysticism meets modern medicine in this intriguing r cit of a nun's dark night of the soul. It's 1997, and Sister John of the Cross, a Carmelite nun in a monastery just outside Los Angeles, seeks treatment for epilepsy, although the remedy threatens to diminish her formidable spiritual powers. The Carmelites place heavy emphasis on prayer, and over the years this discipline has helped Sister John to develop miraculous visionary gifts. When severe headaches precipitate a collapse that requires medical intervention, Sister John finds the process starkly juxtaposed against her centuries-old traditions: she discovers it's almost impossible to discuss infused contemplation with a neurologist. Is her continual prayer ""hyperreligiosity""?; her choice to remain celibate ""hyposexuality""?; her will to control her body ""anorexia""? Although she accepts a CT scan and its diagnosis, Sister John determines that faith offers a more substantial, meaningful reality. Written with simple elegance, alternating narrative and prayer, the tale is engaging yet maintains a curious emotional elusiveness. A drama centering on the realm of mysticism is bound to be difficult to describe and, like Ron Hansen's Mariette in Ecstasy, this story doesn't aim to render the nun's spiritual life and psyche in accessible terms for lay readers. What Salzman conveys with perfect clarity is that momentary, extraordinary mental state in which physical pain becomes pure, lucid grace poised between corporeal reality and eternity, a state that Sister John desires to prolong for a lifetime. Salzman's talent for calling forth the details and essence of unfamiliar realms is well known: his memoir, Iron & Silk, was acclaimed for its deft rendering of life in China, no less authentic for being written by an outsider. With this third novel (after The Soloist), the author continues to surprise with his unorthodox choices and consistently challenging themes, story lines and characters. Eight illus. by Stephanie Shieldhouse. (Sept.) FYI: The Soloist was a finalist for the Los Angeles Times Book Prize for Fiction.