Bookbub

If you don’t have free storage, free up some. If you have a first-few-hops VPN client app in use, remove that and restart the iPad. If Screen Time / Parental Controls / Focus is enabled, disable that, and try again.

You will want to be the same country or region where the book was purchased,

If you are signed in using the same Apple ID that purchased the book, and have sufficient free storage, and have not enabled Screen Time / Parental Controls / Focus, and have not installed a first-few-hops VPN client app, and the book still does download, contact Apple Support.

If you are using Bookbub for this, have you ask them about the problem?

Do you have the same problem when you buy directly from the Books app on your iPad?