It's referring to the Feynman Lectures on Physics http://www.feynmanlectures.info/, he was presumably involved in the presentation aspects of that (the books I mean, not the site).

I don't know if RPF himself had anything much to do with it, but the typesetting of the /The Feynman Lectures on Physics/ is beautiful.

It features the main column / side column design as discussed, with notes, diagrams and navigation hints in the side column and is a great demonstration of the value of whitespace!