If your plus and pro books disappeared with the update, they’re not gone forever. Within the app, click on Options – then at the top right corner, click on settings. The settings screen will pop up, and at the bottom of the screen is the Restore Purchases Button. Once you click the Restore Button, you will be asked for your iTunes password. Once you enter your password, you will get a pop-up that says:
All previous purchases have been restored. If you have downloaded books on a previous version, you will still need to navigate to those books and add them to your library. We sincerely thank you for your continued support.
As the message says, you will need to re-download the books that were purchased previously, but you will not be charged for them.