Mathematics textbooks are a significant pedagogical tool, particularly in light of the growing interest in "flipped" classrooms. However, there has been little research on how mathematics students read and comprehend textbooks. This article uses the ideas of the implied reader, sense-making frames, and sense-making gaps to analyze students' reading of a section of a calculus textbook. In order to distinguish potential weaknesses in students' content knowledge from their reading abilities, the article also compares students' reading strategies to the reading strategies of "expert" readers: professors in technical fields other than mathematics. [For the complete proceedings, see ED583608.]