Textbook research as scientific research: towards a common ground on issues and methods of research on mathematics textbooks

School textbooks have received increasing attention in the international research community of mathematics education over the last decades. Nevertheless, mathematics textbook research as a field of research is still at an early stage of development, and its philosophical foundations, theoretical frameworks and research methods for disciplined inquiry are still lacking or fundamentally underdeveloped. To this end, by drawing on the literature and the author's own work, this paper provides a critical analysis of contemporary issues and methods for mathematics textbook research. Based on the analysis, the paper puts forward a conceptual framework which conceptualizes textbooks as an intermediate variable in the context of education and hence defines mathematics textbook research as disciplined inquiry into issues about mathematics textbooks and the relationships between mathematics textbooks and other factors in mathematics education. The paper argues that researchers need to expand research issues from descriptive issues such as how a special topic is treated in textbooks to correlational issues and, especially, causal issues including how they are affected by other factors and how they affect other factors concerning education. For this purpose, researchers must also go beyond textbook analysis, textbook comparison and textbook use to a new and shifted paradigm of research that employs more empirical and experimental methods and view more rigorously textbook research as scientific research.