You should note that Dark Mode within the Books App doesn't work for all types of document - such as some basic PDF files.
As a diagnostic exercise, try downloading an Apple User Guide (these cost nothing to download). Just enter "Apple User Guide" in the Search dialogue to find an appropriate book to download.
Open the downloaded User Guide from your Books Library, tap an empty area of the screen, then tap the icon at bottom right of the screen to access the Themes & Settings dialogue:

Next tap the button illustrated here:

Finally, select the viewing mode (Light, Dark, Match Device or Match Surroundings) that you prefer. Match device will follow your iPad's device settings (as set in Settings > Display & Brightness > Appearance).

NB: All of the above screenshots were taken using an iPad Pro M1 with iPadOS 18.1 installed.