I rebooted the iPad, and signed out and back into my iTunes account within the iBooks app – still no change and the same error when trying to buy a book. Just for grins, I tried buying an app in the App Store and a single in the iTunes Store. Both of those purchases went through just fine – leaving me a little baffled, as this seemed to rule out both a general sort of server issue on iTunes and a general problem with my iTunes account.