Words are selected in a weird way in iPad Pro Books!
Reinstall Apple Books
Make a backup of all of your files. PDFs, books and anything you have imported into Apple Books. You already have a backup if you use iCloud and Apple Books and iCloud Drive are enabled on Preferences &gt; Your Name &gt; iCloud.
Delete the Apple Books app and download it again from the App Store. Then try again and see if it fixes your issue.
Beta software
I see you are running iOS 14.5. iOS and iPadOS 14.5 are currently beta operating systems and are expected to have bugs and other random issues. I can't guarantee you this specific issue is caused by the beta, but it's possible. I would test it but since I'm a developer I'm also running the developer beta.
Nonetheless, all of the notices on using beta software from this answer apply.
If you didn't have this issue before, you can downgrade your iPad to a stable version and see if downgrading fixes it.
I'll summarize the downgrade steps but you should check out the complete downgrade tutorial here.

Make sure you have a backup of all of your data. If you don't have backups, you will lose all of your data during the downgrade. Backups created on iPadOS 14.5 can't be restored on iPadOS 14.4, which is why Apple tells you to backup before installing the beta. I don't remember if there's a way to keep the data when downgrading without a backup.

Connect your iPad to your Mac or PC via USB.

Open Finder or iTunes and select your iPad.

Force-restart your iPad and enter recovery mode.

Select Restore in Finder or iTunes.

Wait for Finder/iTunes to finish downloading the stable version of iOS to your computer. It will automatically begin installing after it finishes downloading.

Click Ok to the “Your iPad has been restored to factory settings…” message, and wait (don't disconnect the device) until it shows you what are your options for restoring your data.

Restore your data.


If you don't have a backup, you can also wait and check if it is fixed on the next betas. Developer betas are released roughly once every two weeks. You should report this and any other issues you find to Apple using the Feedback Assistant app so they can fix it.

It's best if you report it directly from your iPad, if possible right after the issue occurred, because the iPad will collect logs and diagnostic data that is critical to finding the bug. You can see your bug reports from the web at https://feedbackassistant.apple.com
I think this is likely a software issue (even if not beta-related), but I'll include two other easy things I've thought of which you can try.
Generic cable or charger interfering with the screen
Generic cables and chargers can interfere with the touch on your screen and create weird scenarios when using your devices. Try unplugging them and distancing your iPad from the charger and see if this fixes your issue.
Too much grease interfering with the screen
If the screen is way too greasy (a lot, like picking-up-greasy-food-then-immediately-using-the-iPad greasy), it would be possible for the grease to interfere and keep registering as a touch input on the screen after you lifted your finger. iPadOS has many software optimizations to ignore unintentional touches so this issue could be sporadic and closing the app like you mentioned (or just switching to the Home Screen and then back to the app) would fix it.
I'm mentioning this because there are parents with children who share iPads. If the issue remains after cleaning the screen, then this is not the cause.