"Oh good!" I said to myself. "I can just restore that Books folder with the one on my external drive! But it's been awhile since I backed everything up--better do that first." So I selected everything on my desktop, dragged it to the external drive, and let it do its thing.
A warning box came up that said, "A file already exists with this name." I clicked "replace," then clicked in the handy "apply to all" box, knowing much of what is on my desktop is already on the drive and not wanting to have to click "replace" a million times.
'Course, what does that mean I replaced? The uncorrupted Books file with the corrupted one.
Just how stupid can I get? Not sure; research is ongoing.