The academic textbook is somewhat of a blurred genre. It is the dominant genre of formal education, and its main communicative purpose, according to Bhatia (1998), is to "make accessible established knowledge in a particular discipline to those readers who are being initiated into a specific disciplinary culture" (p. 17). Thus, a textbook is generally seen as a written text that introduces newcomers to a specific discipline.

Bhatia (2004) on the textbook as a genre (click to expand/contract)

"Textbooks [...] disseminate discipline-based knowledge and, at the same time, display a somewhat unequal writer-reader relationship, with the writer as the specialist and the reader as the non-initiated apprentice in the discipline, or the writer as the transmitter and the reader as the recipient of established knowledge."

Bhatia (2004: 33)

As pointed out by Hyland (2009, p. 113), textbooks rely heavily on other texts. He argues that their value hinges on them "representing the issues, ideas, current beliefs and chief findings of the discipline by borrowing and incorporating these from the original sources". 

One postulated trademark of textbooks as a genre is the way authors try to order variation, different perspectives, and even controversies in the field, in an attempt to iron these out in a voice of authority (Hyland 2009).

Another alleged trademark is the relative absence of explicit references to earlier published work. This is so, since the main purpose of textbooks is not to present new, cutting-edge claims, but rather to outline the current, widely agreed knowledge and methods of a discipline. For this reason, linking these to their sources is argued to be of less importance (Hyland 2009).