The Mapmaker's Wife by Robert Whitaker

In the mid-1700's a French scientific expedition sets out for the Amazon, intending to measure the size of a degree of longitude at the equator and settle a debate about the shape of the Earth. They stay for more than a decade, and some of them get more than they bargained for.
One of the expedition members marries a Peruvian woman. They are separated for decades, and she undertakes an amazing journey so that they can be reunited.
It's full of interesting details about life in the South American colonies, political intrigue, oldskool science, and big snakes. Especially interesting if you're planning to travel to Ecuador (like some folks I know) and will get to visit many of the measurement sites.
PS, don't confuse the author with Roger Whittaker. Different guy.