This was a fix that worked for our users, thanks to OkadaBooks' CTO @jude.
"In the interim I have made a minor update on our backend that would help resolve this issue. Please take the following steps to resolve the issue
delete the previously downloaded books
go to the account screen of the app. Stay on this screen for a few seconds for you account information to update
go back to My Book screen of the app and re-download the books
Please help with confirming that this resolves the issue you encountered.
A proper fix will be deployed as soon as possible"