I would suggest a few things. First close all the apps with this article:
iOS: Force an app to close
- From the Home screen, click the Home button twice.
- Tap and hold on the app.
- When it starts to jiggle, tap the - to close it.
- Double-click the Home button and try opening the app again.
If the issue persists I recommend restarting the iPad:
iPhone, iPad, iPod touch: Turning off and on (restarting) and resetting
Restarting your device
- Press and hold the Sleep/Wake button for a few seconds until the red "slide to power off" slider appears, and then slide the slider.
- Press and hold the Sleep/Wake button until the Apple logo appears.
And as a last resort you can delete the book and download it again free of charge using this:
Downloading past purchases from the App Store, iBookstore, and iTunes Store