Textbooks are a standard component of undergraduate mathematics courses, but research shows that students often do not view textbooks as productive resources to support learning. This article seeks to understand the factors affecting how individuals engage in reading a calculus textbook excerpt and what they learn from reading. To better understand the separate roles of background knowledge and other reading practices, we compare 2 readers: a 2nd-semester calculus student and a nonmathematics STEM professor. We employ the concepts of "sense making" and the "implied reader" to analyze each reader's experience and a disciplinary literacy perspective to explain the similarities and differences we find between the 2 readers. We propose the concept of "didactical disciplinary literacy"--an adaptation of disciplinary literacy applied to didactical texts--to describe the ways that the professor drew on his identity as a teacher to shape his reading practices.