Kernewek Dre Lyther (Cornish By Letter) ("Cornish by email" nowadays, of course).
I later got a couple of textbooks (Bora Brav, Skeul an Tavas, Desky Kernowek) to supplement that but KDL was my main tool.
Extremely handy if you can't attend a face-to-face class; much better to have a real teacher at the other end of an email who can answer questions and correct errors before they sink in, compared to just working by yourself from a book with no feedback beyond the answers at the back.
You are the second person to tell me about that. I probably ought to look into it. Thanks