I liked Longitude and would recommend Galileo's Daughter, also by Dava Sibel. It's a biography of Galileo supplemented with letters he exchanged with his daughter, who was a Catholic nun.
As an aside, if you're ever in Florence, Italy, pay a visit to the Galileo museum. You'll get to see his desiccated fingers and a huge collection of medieval astronomical instruments that were as much art as science. Galileo's experimental devices looked a lot like things you'd see in a high school today, except made from beautiful wood and brass. It really blew me away.
Note: he had two daughters, both nuns. (Which was really the only option for unmarried women at the time.)
It's just that the letters were only with one of them.