The River of Doubt by Candice Millard is a solid book subtitled Theodore Roosevelt's Darkest Journey. Millard chronicles well the ex-Presidents 1914 voyage down an uncharted tributary of the Amazon River and it was a remarkable tale of a harrowing journey.
The co-commander of the expedition was Colonel Candido Rondon, a man Roosevelt respected greatly, and a hero to Brazilians. The fierce determination of Roosevelt to the mission of the expedition at the cost of his personal safety was striking, along with how dangerous it was. They spent months in the wilderness, dealing with disease and near-starvation. They continually lost their canoes, both ones they brought and ones they made, replacing those lost or not suitable for use on the perilous waters.
Roosevelt is a fascinating character. He was an enfeebled child, and built his body up through determination, action, and hard work, things that became a constant for through the years. Roosevelt undertook the Amazon journey in part to try to get over a failed presidential bid, with the loss to Woodrow Wilson leaving him doubting himself. He went to Brazil with his son, Kermit, and the expedition was wildly optimistic starting out, with provisions way too heavy and selected for comfort and enjoyment.
The actual route taken was not the original plan of going down a well-known river. It was Brazil's minister of foreign affairs who proposed the path down the unknown river, knowing that was the sort of thing that would appeal greatly to both Roosevelt and Rondon. Except for the indigenousness tribes, only a few people in history had ever reached the headwaters of what would become named the River of Doubt. The expedition faced grave danger from the local tribes, and Rondon made a point of trying to have relationships with them, even if that meant he or his men be killed. He refused to let his men retaliate against the Indians as he felt forming these relations even more important than exploration. The men often had to portage around rapids, cutting through the jungle and carrying everything. Then the rapids they did try to pass through could be exceedingly dangerous, and led to to the death of one of their crew. Another of their crew killed a fellow member, and then was almost certainly killed by the Cinta Larga Indians as he abandoned the expedition.
When Roosevelt was on the brink of death from malaria and a bacterial infection as the result of a gash on his leg and considering ending his life to ease the burden on the expedition, it was his desire to see Kermit, one of his sons, survive the voyage that kept him going. They spent six weeks on the river before seeing signs of non-Indian life, they came across- rubber tappers, those who took from the Amazonian rubber tree. Roosevelt lost 55 pounds on the journey, a quarter of his body weight.