Textbooks have been the primary teaching tool since the 19th century. By their nature, they contained a comprehensive compilation of the content of a particular subject with the intention of explaining it; this knowledge, in turn, was usually filtered to conform to a particular society's expectations of elementary knowledge about the natural and social environments. There has been a great deal of research on how the content of textbooks has changed in line with changing values in different societies and over different periods. However, little research has been done on how textbook reading substrates and design have changed and how these changes have affected learning and comprehension: studies that systematically examined the effects of different reading substrates and different layouts on reading and learning comprehension did not appear until the late 20th century and early 21st century. We examine such studies and PISA 2021 results to draw five conclusions for future textbook research. These conclusions indicate that screens are worse than printed texts for some types of reading, while interactivity and dynamic design are not values per se but require coherent design to improve reading performance and higher-level thinking skills.