Book file must have been corrupted. I built a new book file--the documents inside were and are fine. The BookHTML reference page has appeared. I have hand-edited the first document's HTML mapping table, then copied it to that document's BookHTML table (it overflows the text box that's already as big as the page, but all the data is there even if I can't see it). Resulting HTML conversion works as expected. Woo-hoo!
--Karen