Now I can change text in any text box except the one titled "spinetext", which is highlighted in the layers panel. Obviously I'm doing something wrong, but I've designed a lot of these covers, and never had trouble changing the spine text before.
I finally had to recreate that layer in a new document and import it into this one -- but the text remains unchangeable in this layer.
(For what it's worth, "Layer 1" is the template, which I turn off when I have inserted all the elements, so it has nothing to do with this layer.)
Thanks for your help!