As a practical consideration, one might be able to wrap a cover around a book. If one is peering over your shoulder, an ebook might be advised with a privacy screen.
Until the airline crew conclude that hiding the title of the book one is reading is equally suspicious. After all, innocent people have nothing to hide, right?
Use a cover depicting flowers or palm trees, nobody will suspect anything--or as the poster suggested via YouTube video, a cover with the title "Ass Eating Made Simple" /s