Reason-based understanding is a powerful kind of learning and is often a desired outcome in the classroom. When it is a goal, textbooks which concern themselves with this kind of understanding may offer the teacher subject and pedagogical knowledge and help students achieve more. Such textbooks may be described as models of practice for these teachers. Some models of practice are likely to be better than others. This article describes a procedure for identifying textbooks which, on the face of it, have the potential to support students' reason-based understanding and help teachers do likewise. The procedure involves counts of cause and purpose clauses in samples of text and the classification of the clauses into groups. This can contribute to the selection of textbooks with the potential to support this kind of understanding and be useful models of practice for teachers.