I assume you are referring to "Endurance: Shackleton's Incredible Voyage" by Alfred Lansing. There is also a very good account in "The Endurance: Shackleton's Legendary Antarctic Expedition" by Caroline Alexander. Amazing tale of fortitude and integrity.

Regarding the journey across Elephant Island, that too was incredible. I note that there are tour companies providing opportunities to do this, though I haven't managed to do it yet.