If you try downloading a book on the app and you get an "Access Denied" error then you might have disabled permissions for the OkadaBooks App on your Phone

Go to Settings -> App Settings -> OkadaBooks App or Press your finger on the OkadaBooks App icon on your app drawer

Then click on App Info like in the image below:

Then click on Permissions and enable (turn on) permissions for Storage and Location.

Then try downloading the App again.

If the problem persists then uninstall the App and Reinstall it and try again.