Thanks again! It was the book descriptions I was trying to read. It turns out I had simply missed a step in changing this in "look and feel." I'd clicked the apply button in the font changer, but had neglected to click the check on the original look and feel pane. My bad!
Kovid, I will definitely remember you again the next time anyone pays me for anything (doesn't happen all that often, alas.) I think Calibre s probably the best supported free software out there.