OK, I opened another book, make some formatting changes to it, then went to File -> Save As and saved it somewhere else. The default was an HTML format.
I worked on another book.
Then I went back to my original book. While some of the formatting appears to have stayed, I lost a lot of the BookDesigner features.
All of my page breaks are gone.
Some of the text that was originally a titled and I formatted as a paragraph is back to a title.
How am I supposed to save my work and come back to it later?