As John Rolfe Gardiner's gripping and elegant new novel opens, World War I is raging and letters home from Major William Lloyd describe his life as a volunteer doctor in charge of a base hospital in the "zone of advance."         The Major's new command is both dream and horror to this upright son of an old New York family, bred to Victorian virtue and duty. Supplies are erratic, sanitation abysmal. Some of his nurses are behaving like trollops and an old prep school enemy turns up as his adjutant.         On the home front, the doctor's anxious wife, Emma, has troubles of her own. Her daunting mother-in-law has moved the family to her Long Island estate to escape city germs. Her two sons, one of enlistment age, are developing alarming pacifist sympathies, and the flag-waving chauffeur is spreading rumors about them. Her teenage daughter is growing up too fast.         But it's the doctor's correspondence from "somewhere in France" that most disturbs, with its frequent mention of the remarkable French nurse, Jeanne Prie. We hear about her devotion to the patients, her doctor-like authority, her revolutionary work with victims of the dangerous unknown fevers spreading through the trenches. We learn, too, of her mysterious origins, of claims that she was an assistant to Louis Pasteur, of the aura of suspicion and wonder that surrounds her.         Gradually the doctor's obsession with Jeanne becomes clear to everyone but himself. And when his son is drafted and follows him to France, and when the nurse's audacious experiments involve her in controversy, the situation spins out of control, forever changing all their lives.         Somewhere in Franceis a riveting tale of medical suspense, a portrait of a society in transition, and an affecting love story that explores the mysteries of trust and faith.