I was inspired to read the book in part because of the recent and critically well received movie of the same name that was based on this book (I haven't seen the movie).
David Grann, whose main profession is as a journalist for The New Yorker magazine, became fascinated with the British explorer Percy Harrison Fawcett and his disappearance in the Amazon around 1925. Fawcett became convinced there was a fabulous lost city in the Amazon, very similar to the idea of El Dorado. Many, many people have followed in Fawcett's footsteps over the decades since, but it seems that Grann, through a combination of dedication, luck, and slightly more objectivity than most Fawcett fans, got closer to the truth than anyone had previously.
Grann jumps back and forth between Fawcett's personal story, the process of his own research, and his own trip to the Amazon, to present a fascinating picture of the time period of Fawcett's exploration and of the Amazon itself. He explains about Fawcett's competitors, his previous expeditions, and his family (some of whom Grann ended up meeting). I was pretty sure the book would end without finding out what happened to Fawcett or ever finding the fabled city, and I would have enjoyed it even if that's how it had wrapped up because Grann built up such a detailed and interesting picture. Fawcett's final fate hasn't been determined (it probably never will be). But to my surprise, they now have a pretty good idea what inspired the legends that Fawcett was pursuing - and explain quite clearly why he could never have found it, even though it was there.