The Third Man Factor
I read a short review of John Geiger's book, The Third Man Factor and immediately caught the reference to Shackleton's miserable trek across South Georgia seeking help for his men.
I had no idea, however, that there have been so many incidences of "third man" appearances in the twentieth century. Geiger collects many of these stories from the survivors themselves, and presents them and the "scientific" theories and religious theories that seek to explain them. Most of those who experienced the help and encouragement from a being not actually with them were convinced that the help was angelic. A few insist that the phenomenon was simply a part of their brain desiring survival kicking in and making them manufacture imaginary helpers that were really themselves.
It was an interesting book, and one I enjoyed.