I am a backup kind of gal and added the sqlite file stored on my desktop to the things I have set to automatically back up each night. A few days ago, as I dragged a new purchase downloaded with ADE to the Kobo icon, the little signal light was blue, rather than green (not a Kobo problem, I think, but rather due to a strange periodic hiccup on my computer). Sure enough, the book did not end up in my reader, but as I played around, I came to realize that there was a pointer to it in my sqlite file. That kept me from being able to load the book from ADE again and have it show up. From past experience, I knew I could have sideloaded it through Calibre and then I would be good to go, but instead, I went to my backup and replaced the now wonky sqlite file with the one from the day before. I then downloaded the book again, and presto chango--there it was.
I'm throwing this out for folks who might not want to start over with an entirely fresh sqlite file and who. likeme, backup the desktop sqlite file.