Readers never have to search for or carry a separate light for the hardcover or paperback book they're reading. Simply slip the book into the Periscope bookcover (there are separate covers for hardcover and mass market paperback books), pull up the retractable light that's stored in the spine of the book cover and start reading in the dark! The Periscope adjustable twin, wide-angle LED light automatically turns on when the arm is extended and shuts off when collapsed into the cover. What’s more, because the light is totally adjustable, readers can put the reading light precisely where they want it for maximum reading comfort. And, because Periscope is battery powered by three AA batteries that deliver over 40 hours of continuous light, there are no cords or plugs to worry about.