Thank you both for your advice. I'm glad that 500 books isn't seen as nearing any kind of limit.
In the meantime, frustrated and impatient (did I wait 5 minutes? Or maybe 10?)
I started looking at other readers and ended up installing three others, none of which was satisfying, one because the system was impossible, or (for the other two) they only loaded about 240 from 500+ into the bookshelves.
I ended up doing what you have suggested and what I should have done first: shut down and reopened, then I uninstalled Moon Reader, shut down again, reopened and reinstalled. Voila! Everything OK again. I didn't BTW delete the books, I left them there and it worked.
I'm actually thinking of laying out the money for the Pro version, but I'm unsure if it offers me anything better than the free (except no ads, of course).
Thanks again for taking the time