what sorts of background texts/sources of info did you use?

edit: i think i have the dover book your readme mentions :)

Initially (about 15 years ago - it was my first ever open source library!) it was based on Astronomy Answers articles that are thankfully still live at https://www.aa.quae.nl/en/reken/zonpositie.html, but v2.0 specifically was rewritten to use the approach described in Jean Meeus' Astronomical Algorithms book.