In this paper, we propose an analytic tool for describing the mathematics made available to learn in a "textbook lesson". The tool is an adaptation of the Mathematics Discourse in Instruction (MDI) analytic tool that we developed to analyze what is made available to learn in teachers' lessons. Our motivation to adapt the use of the MDI analytic framework to textbooks is to test the relative robustness of the framework in moving across different pedagogic texts (e.g. video of a lesson, a textbook lesson). Our initial findings suggest it has applicability across pedagogic texts, thus opening possibilities for using a common framework and language in research and in professional development activities involving the written and enacted curricula.