We investigated university students' engagement with digital textbooks by analysing their real-time viewing and use in a linear algebra course taught at four different universities in the United States. Viewing data over a semester was complemented with responses to bi-weekly open-ended surveys and other course artefacts, such as course syllabi. With our approach to ask students about their viewing data in surveys, we were able to relate student majors to textbook engagement and to document more and different uses of textbook elements than what has been reported so far. We used descriptive statistics and frequency graphs to analyse the viewing data and a grounded analytical approach to survey data. By using the didactical tetrahedron, we found that textbook engagement dropped when the algebraic language and content of the textbook was not institutionally legitimized and when the textbook did not directly satisfy interests associated with earning a degree. Textbook engagement was higher than what could have been anticipated by syllabi for students majoring in disciplines other than mathematics. We propose areas for further research.