At a time when it is seen as increasingly "obsolete," this article analyzes the textbook as an evolving pedagogical form, as a changing medium comprised of smaller media components. These components include images, diagrams and also oral prompts, which have changed not so much through technical innovation as in synchrony with larger cultural and epistemological developments. This article investigates the increasingly sophisticated structuring of this textual and visual content, and the gradually sublimated "oral" interaction simulated through cues and interrogatives. These components have become highly conventionalized and elaborate, characteristics generally ignored to the detriment of publically-funded "open" e-textbook projects. Following Thomas Kuhn's famous analyses of knowledge "paradigms," this article concludes that the textbook's features provide an indispensable animating didactic function.