Passing over the four photographers I've already heard of; Robert Doisneau, Marc Riboud, Bill Brandt and Ansel Adams I've chosen this photo by Philip Jones Griffiths (1936-2008).
Griffiths joined Magnum as an associate in 1966 becoming a full member in 1971.
I chose this photo because it draws me in to ask questions. Why is a woman in a graveyard staring so intently at some black thread? Looking closer you can see the hand of a man in the bottom-right holding onto the same thread. What's the back-story here? There's no detail on what it features other than to say it's a girl in Brompton Cemetery. I want to know more, and that's half the battle with making an interesting photo.
You can read more on Griffiths over at his Magnum Profile Page.
It isn't great quality, but here's an interview aired in 2005.
Unrelated to the above but I was pleased to find the original receipt for the book had been used as a bookmark and remained in the book.