Here are some excellent resources.
Parts 2-5 of this BBC series start here (part 1 not available): https://www.youtube.com/watch?v=AxGbRFw1dNE
https://en.wikipedia.org/wiki/Klezmer
https://www.klezmershack.com/articles/aboutklez.html
https://www.iemj.org/en/klezmer-music-from-the-past-to-the-present/
http://borzy.info/EnglMCKlezmer.htm (warning: my computer said not safe but it worked fine when I bypassed that screen)
https://www.safed.co.il/klezmer-history.html
https://archive.org/details/Beregovsky1941/page/n7/mode/2up
https://www.natseelen.com/courses/scholarship
https://www.natseelen.com/courses/
http://www.christiandawid.com/about.html
https://bern.academia.edu/JoelRubin