Thanks to everyone here!
To conclude - I did a log in/log out, followed by a manual update.
Unfortunately, none of it worked, so I had to do a factory reset afterall.
Still, that seems to have fixed the corruption, and I have recopied all my books across.
I also took the opportunity to patch the firmware, something I'd been meaning to do for a while.
I am having a couple of issues - I don't know if they are due to the patch or due to the new firmware, or due to something else. I'll do some research and you might see me back here again with new questions, or asking over in the developers corner
