You can turn e-book mode on or off in any of the following ways.
Slide out the notification panel from the status bar at the top of the screen, continue to slide out the entire shortcut menu, and then click Book to turn e-book mode on or off.
If you can't find the e-book mode switch in the notification panel, click Edit and drag it to the display area of the notification panel.
Go to settings > display and brightness to turn the e-book mode on or off.