If you are interested in esotericism, you are interested in philosophy (whether you know it or not). PhilPapers and PhilArchive is a pretty huge online collection of articles, books, etc. on the subject of philosophy writ broadly. Not everything is immediately downloadable, but you can import citations in many formats. Try search terms like ‘esoteric’ or ‘Pythagoras’ and you’ll find all manner of interesting stuff.