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:

https://science.nasa.gov/learn/basics-of-space-flight/

http://www.braeunig.us/space/orbmech.htm

https://spsweb.fltops.jpl.nasa.gov/portaldataops/mpg/MPG_Docs/MPG%20Book/Release/Chapter7-OrbitalMechanics.pdf