The search box, topright, and the LeftSideBar links to previous lessons seem to me to be adequate, but -- FWIW -- perhaps in-line links to specific previous discussion might have occasional merit?

Hard to know, of course, what criteria to use for backrefs of that sort. Too many = clutter and additional filesize + writing time for you.

Observations from one who owes much of his understanding of CGI to your course. ++!