First try a soft reset on the iPhone.
Restart your iPhone 8 or earlier or iPod touch
Press and hold the Top (or Side) button until the power off slider appears.
Drag the slider to turn your device completely off.*
After your device turns off, press and hold the Top (or Side) button again until you see the Apple logo.
* With iOS 11 or ...
Dr Google says...
Download Titles Again
If you are still having issues with the downloaded titles, be it blank pages or wrong book, delete the downloaded book.
Next, go to your account’s purchased section under Reading Now or for iBooks, visit your account settings in the iBooks Store and re-download the title.
This likely solves your issues even when you ...