To follow up on my last post, after I ran CHKDSK with the /F switch, the directories were readable and all was fine. But when I unplugged it, it tried to synchronize and still found no books, and when I plugged it back into the computer, it was once again "corrupt and unreadable."
It eventually dawned on me that in its attempts to synchronize, it was corrupting itself. So I ran CHKDSK /F again, then went into the directory, backed up all my ebooks and deleted every ebook file from the e-reader, including the ones in the Digital Editions directory. Once I'd done that, I ran Kobo Desktop, and it started downloading the new firmware upgrade, so that was a good sign.
After the upgrade was done, the reader was working but still had no books on it, so I had to figure out how to get the 100 preloaded books back on. Happily, that solution was available right on this forum:
https://www.mobileread.com/forums/sho...7&postcount=12 (thanks Sameer). So I followed those instructions, and then all my books were back, and I was very happy--and tired, because it was 1:30 in the morning.