Many ebook readers solve this with an extra set of page forward / page back buttons on the left. (activitypress.com)
If you no longer want to have a screen lock, just press back button to exit the Security screen. (phoneradar.com)
Your thumb naturally rests against the large «next page» button, while the smaller, less - used back button is given less space and requires a bit more effort to press. (trustedreviews.com)