How do I turn off dark mode in Books?

Light/Dark mode for the Books App is controlled from within the App...

1) Starting with a book being open, tap the button at the extreme bottom-right of the page; if the button is not visible, just tap the open page with your finger:

2) Next, select Themes and Settings from the menu:

3) Next, tap the mode button - and from the menu select Light, Dark or Match Device as preferred:

Note. If you select Match Device, the mode will follow the system Light/Dark mode in iPad's main settings:

Settings > Display & Brightness > [Appearance] - select Light, Dark or Automatic

Sett > Display & Brightness > (Light Mode)

If only material in the Books app appears in Dark Mode, perhaps Settings > Books > Automatically Invert Images = “off” will help.

You're very welcome. I'm delighted that the problem has been successfully resolved.