[GeoffR]

I don't backup anything from the device itself, but I have all my original books in Calibre, and I occasionally add a flag in Calibre to the books I have read recently. If I had to do a factory reset then it would just be a matter of sending the books from Calibre again, and using the KoboUtilities plugin to set the reading status to finished for the books I have flagged as read.

There is no other information on the device that I care about enough to bother backing up. (I don't make annotations, except for temporary ones to mark errors which I delete immediately after correcting the original book in Calibre.)

Edit: If I had lots of devices or was in the habit of doing frequent factory resets, then I would also backup the .adobe-digital-editions directory to avoid the need to reauthorise the device after a reset.

I also have the original internal SD card and a backup from when I opened the device to upgrade to a larger card.