In 1916, while the marooned Sir Ernest Shackleton and two crew members fought fiercely for survival on South Georgia, they had the sensation that another, providential person accompanied them that they “were four, not three,” as Shackleton wrote. This phenomenon of sensing a beneficiary presence during extreme tribulation became known as the Third Man Factor, and everyone from Reinhold Messner and Joe Simpson, to Charles Lindbergh and Aron Ralston, to astronauts and 9/11 survivors has reported it. In The Third Man Factor ($24.95, weinsteinbooks.com), John Geiger expertly blends harrowing accounts with scientific inquiry into the physical, neurological, and psychological elements common to these events. People have suggested myriad causes, from a guardian angel, to lack of oxygen, to an evolved coping mechanism. Whatever the case, Geiger offers a thorough, insightful examination.