Purchasing a book

Try the below, stop when it works.

Force close the Books App: link-> Close an app on your iPad - Apple Support

Try again,

Restart the iPad: link-> Restart your iPad - Apple Support

Try again,

Reset your Wi-fi router.

Try again.

Try a different Wi-Fi Network

Try again.

Make sure the Wi-fi network you are connected to is not blocking the store address.

Impossible to answer without any details.

What trouble are you having exactly? What happens when you try to buy it? Do you get an error? If so what does it say?

Are you trying to buy it from the Apple Books App or from some other book store like Amazon, or Google Play Books App?