Apple Books freezes up repeatedly

I have purchased Mac OS Catalina by David Pogue from the Apple Bookstore. I've been reading along, highlighting here and there, and suddenly, last night, it wouldn't let me do anything: highlighting, turning the page, etc.

Tonight I picked it up to try again and it let me turn the page once before freezing up again. I have tried quitting and relaunching the app, rebooting the iPad, making sure I am updated to the latest version -- 13.5.1 (17F80), etc. Advice appreciated.