HOW TO Make an iPad a dedicated e–reader
Clear space for content
To maximize space for more books, clear out your device by doing a fresh install of iOS (Settings > General > Reset > Erase All Content and Settings), then reinstall your book apps and download your content.
Cut out distractions
Go to Settings > Notifications and turn off notifications for any remaining apps. Now go to Display & Brightness settings and turn on True Tone and Night Shift. Finally, open Control Center and turn on Do Not Disturb.
Sync to cloud storage
If your iPad is really lacking space, use a cloud storage service such as Dropbox. For iCloud Drive, go to Settings > (Your name) > iCloud, and turn on iCloud Drive and Books. Next, go to Settings > Books and turn on iCloud Drive.