I am not aware of sources. I have taught on-line and my #1 concern was “tone”. Words, phrases, jokes, etc. can come off entirely different on-line than when spoken. Be especially concerned about…in fact, avoid….dry humor.
My best results, discussion wise, was when I could share real life situations.
If a student asks a question, throw it out to the class to answer if time allows. Be sure to always answer the question yourself…eventually.
Also, the publishing standards here at Permies has some excellent ideas on non threatening communication.