This is an attempt to place the problem of textbook theory into a new frame of reference provided by cultural-historical theory of development, using the idea about cultural-historical mediation of individual development and concept of CST as a basic analytical concepts. The new frame of reference introduces a new language on textbooks, opening a range of new research problems, while dealing with the old ones in a new and more complex way. Textbook is shifted from the usual pedagogic-didactic setting and placed in a more basic and cultural context. The paper is an attempt to bring the concept of cultural tools to a completely operational level, as well as to test the actual power of that concept by identifying CST in textbooks. Furthermore, this paper is an attempt elaborate the forms of social assistance that constitute the zone of proximal development, seen as the characteristics of joint activities of the child and a more competent participant (which is "hidden”, i.e., indirectly present, in this case). Vygotsky never specified the forms of social assistance, but rather gave general prescriptions (Moll, 1990), thus presenting one of the central theoretical and practical tasks for the followers of his ideas.