University practice allows the lecturer to recommend the students additional study material.
Before the web, that term referred to monographs, scientific monographs, essays, problem texts and technical books.
The most popular materials appearing in large numbers on the web are different reference books, dictionaries, lexicons and encyclopaedias. Web reference books are a good example of how a new form of expression like hypertext material influenced the structure and organization of the content of a subject that is being taught.
These materials are organized by particles of knowledge in a way that is adapted for the web. The manufacturer of one of the best professional programs for mathematics supports a small encyclopaedia of mathematics, physics, chemistry and astronomy. This site is growing in an almost classical manner; anyone can contribute to the content of this excellent encyclopaedia through the web.
Reference books cover different areas from design, stomatology to biology. In many cases their content cannot be directly related to just one subject described in the curriculum, so they cannot be classified as university textbooks in the classical sense of the word. But too much confidence in computers and programs can end badly as well. A reference book on crystallography rich in examples becomes comical when translated to another language through the offered computer translators.
Since web offers a selection of extremely good quality data from renowned and reliable sources, special attention must be paid to the data reliability. Some world-known publishers allow some of their publications to be available in an electronic version.
Reference books should be taken as data bases of knowledge particles which can be used in their current form. Some authors might use that data to create text-books according to curricula.