E-books generally refer to digitized publications. They can include words, pictures, sounds, images, or a combination of two or more. Now people often read e-books with smartphones, tablets, or e-books readers that are specially made for e-books. E-books have colorful contents and are convenient to store. Their prices are cheap and they are conductive to the popularity of reading. E-books are traditionally believed as an electronic version of the printed books. but now many e-books exist without any printed equivalent. E-books are stored digitally, so they can be read at any time. In 2012,e-books sales in the United States surpassed hardcover book sales for the first time.