Thank you, Stinger.
I'd actually tried "all of the above" over the past day, but I tried them again with no luck.
You're right -- it was the sqlite file. (I looked into that file today using an sqlite editor and saw that some remnant of the Calibre file was still there.) So, I deleted the sqlite file and re-downloaded all my purchases and it looks like the problem is solved.
Thanks for the help.