I liked https://www.wescottdesign.com/articles/pid/pidWithoutAPhd.pdf https://www.wescottdesign.com/articles/pid/pidWithoutAPhd.pd... though I’ve not looked at

I liked https://www.wescottdesign.com/articles/pid/pidWithoutAPhd.pdf https://www.wescottdesign.com/articles/pid/pidWithoutAPhd.pd... though I’ve not looked at the OP so I don’t know how it compares.