There are many Stevensons behind the initials RLS, but the one that has endeared him to readers for so long is surely the fighter, battling to stay alive. Jorge Luis Borges described his brief life as courageous and heroic. In Philip Callow's absorbing new biography, one can see why. Doctors, called repeatedly to what should have been his deathbed, would find a scarecrow, twitching and alive. A sickly child, Louis became in turn a bohemian dandy,...