The River of Doubt, by Candice Millard

The River of Doubt:  Theordore Roosevelts Darkest Journey

After Teddy Roosevelt, the 26th U.S. president, failed his re-election effort he decided to have a great adventure. It had to be unique and something no one else had done. Exploring an uncharted river in South America fit his needs well. 

The River of Doubt is a black river that snakes through one of the most treacherous jungles in the world. The rain forest was anything but easy to deal with.
He took his son Kermit and they found an experienced guide, Candido Rondon, and crew. 

Candice Millard said in writing about the trip, "When he wasn't too sick to sit up, Roosevelt sought comfort and distraction in the world that he knew best: his library. For his trip to Africa, he had spent months choosing the books that he would take with him, ordering special volumes that had been beautifully bound in pigskin, with type reduced to the smallest legible size, so that the books would be as light as possible."

Vipers, piranhas, poisonous plants, insect swarms were all serious threats as were Indians armed with poison tipped arrows. 

Roosevelt ended his journey sick with fever having lost 1/4 of his body weight. It was indeed Teddy Roosevelt’s darkest journey.