Damn. Seems like this might well be caused by a faulty CF card, I've discovered that about 100 of my books have disappeared!

Not only that, but they must have disappeared before I did the last backup, which means I'm going to have to chase them all back to the websites I found them on in the first place.

I'm going to try reformatting the CF card, and see if that makes it behave itself; I've only had it a week, so it should be working.