Its format may be plain text, HTML, SGML, PDF, or any of a variety of encrypted formats. Unless special restrictive technology is applied, an ebook can be freely copied to computers and from computer to computer and saved on digital storage media of all kinds. It can also be printed on a computer printer and read in paper form. A book is a large and meaningful set of words. It can exist in many forms, both analog and digital, but its ultimate destination is the human mind.
Computers can remember any sequence of characters or code, regardless of whether it has meaning.
So you might say that when it comes to large sets of data, computers can store anything, but humans can only store books. Today, very few people bother to memorize entire books. It takes talent, training, and dedication to do so. In our day, it would probably take the incentive of strong religious belief to accomplish such a monumental feat, like memorizing the Koran, when such easier means are readily available for saving and accessing book content. In the past, whatever could be represented visually could be duplicated photographically.