iPad stuck in Apple Books app, cannot even force quit

I'm stuck in Apple Books on my iPad and cannot return to the home screen.

I've tried:

• Click the home button - nothing happens
• Double-click the home button - nothing happens
• Pinch with five fingers - nothing happens
• Triple-click the home button - brings up zoom, color filter etc., but don't get any further
• Hold the home button - Siri comes up and works, but don't get any further
• Ask Siri to open a different app, e.g. Safari or App Store - Siri says she can't
• Hold the power button to shut down - nothing happens
• Open and close various PDFs and ebooks in the Books app, leaf back and forth, ... - don't get any further
• Let iPad go to sleep - when I unlock, I'm back in Books as before
• Connect the iPad to my Macbook and open iTunes - iTunes waits for me to allow access, but the iPad doesn't even show that message.

Now I can wait for the iPad's battery to run flat, but since the screen turns off after 2 minutes and the battery is still at 90%, that will take hours and require my attention over and over.

Using iPad Air A1435, iOS 12.4 (latest patch)

Do you have any suggestions how to force Books to quit / shut down the iPad?

Thanks in advance for your help.