If it's actually an ocular migraine (which takes place in the eye) rather than a scintillating scotoma (which takes place in the brain), that's a possible explanation for why I could continue reading. The UK's NHS says they tend to happen in just one eye, so if I was having an ocular migraine, it might have left my vision unobstructed in the other eye.

On the other hand, if that's the explanation, then I would sort of expect to have consciously experienced seeing the book while I was reading it, and I didn't--or if I did, that's not the way I remember it.

Of course, memory is untrustworthy, and it's also possible that the vividness of the ocular migraine might have persuaded my brain to tell me I couldn't see the book even though I could.

So ocular migraine is a solid alternative candidate to explain my experience. If I have another scotoma, I'll see if I can figure out a way to determine which thing is going on. Maybe closing one eye and then the other.