Textbooks are the educational input most consistently associated with gains in student learning. Particularly in developing countries, textbooks are the primary conduits for delivering content knowledge to the students and the teachers base their lesson plans on the material given in textbooks. Abstracting from the education literature, we propose that well-written textbooks exhibit the following properties:
FOCUS. Each section explains very few concepts.
UNITY. For each concept, there is a unique section that best explains the concept.
SEQUENTIALITY. Concepts are discussed in a sequential fashion: a concept is explained prior to occurrences of this concept or any related concept. Further, the tie for precedence in presentation between two mutually related concepts is broken in favor of the more significant of the two.