Orbital Mechanics

As part of relearning calculus and physics, I want to explore and master orbital mechanics. I have an idea for a science fiction story in which the world building involves two Earth-like planets that revolve around each other (technically, around their barycenter, which would be located roughly between the two planets). When I took physics in 1972-1973, I knew enough math to be able to calculate a circular orbit of a satellite around the Earth.

Websites providing an introduction to orbital mechanics include: