Edbro, many thanks for the great advice! I have now created new folders in both the internal memory and SD card and that seems to work. (Why doesn't the manual tell you these things?
And thanks, Astra, for the suggestion to use the Ebook Library. If things still go pear shaped, I'll try that. (BTW: I dislike using the Library. Every time I open it, it tries to sell me something...)