You might want to check out Open Math Notes, which is hosted by the AMS and contains almost 100 notes in various subjects, at levels ranging from undergraduate

You might want to check out Open Math Notes, which is hosted by the AMS and contains almost 100 notes in various subjects, at levels ranging from undergraduate survey course notes to advanced research: https://www.ams.org/open-math-notes https://www.ams.org/open-math-notes