Hi RMSRo,
The Dictionary feature is so handy to use, and we know it's working because you have no problems getting definitions when using the Kindle app on your iPad mini. Let's try a few steps and see if it will get the Dictionary working again in Books.
First, we'd like for you to force close the Books app. 1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.2. Swipe right or left to find the app that you want to close. 3. Swipe up on the app's preview to close the app. After the Books app is no longer in the apps previews, we'd like for you to restart your iPad mini following the steps below.How to restart an iPad with a Home button1. Press and hold the top button until the power off slider appears.
2. Drag the slider, then wait for 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.3. To turn your device back on, press and hold the top button until you see the Apple logo.After your iPad mini is back to the Home Screen, open the Books app and test several different books to see if you are able to get definitions.
This should help resolve the issue, but if you still need help let us know.
Cheers!