In this study, we investigated the literacy practices of undergraduate calculus students and non-mathematics STEM faculty members as they read excerpts from a calculus textbook. We found several differences in their practices that appeared to be tied to the didactical nature of the excerpts and the participants' identities and power relationships with the textbook. We use these differences to explore the concept of "didactical disciplinary literacy" to examine the ways readers approach the process of learning from a mathematics textbook and how their practices are related to their positionality with respect to the text. We also explore the development of the readers' thinking about the mathematical concepts and suggest connections between this development and their literacy practices. The study results have implications for the ways that mathematics textbooks can be used as resources for learning the content and literacy practices of the field.