The basic characteristic of hypertext materials are links. Links bind parts of text you wrote with other parts of text, most often through key words, but they can also include pages or sites saved at a server on the other end of the world.
Just like bibliography, books can contain a list of links interesting for the described area of study. Clearly, that presumes the copyright problem as well as the reliability of the information presented.
Hypertext materials almost always include pictures, video materials or animated illustrations. It can also contain audio illustrations.
All those possibilities make hypertext materials content high and suitable for educational purposes.
Hypertext material is stored in an electronic format and its printed version can never be an exact copy.