In many educational systems, textbooks are a leading factor for daily instructional practices, and consequently may affect educational outcomes. Whereas in many countries textbook assessment is a common practice in light of evaluating and improving educational effectiveness, in other countries these practices are absent, mainly under the realm of educational freedom. In this study, we attempt to highlight the potential contribution of textbook assessment to educational freedom, effectiveness, and improvement, by means of mathematics textbook assessment primary education (Grade 4) in Flanders. Multilevel (multivariate) modelling was used, studying the relation of textbooks with both mean achievement (overall and in different content and cognitive subdomains) and (social and ethnic) equity. Furthermore, multilevel quantile regression analyses were used to investigate the effect of textbooks in different quantiles of the achievement distribution. Implications of the results of textbook assessment are discussed in light of its contribution to educational freedom, effectiveness, and improvement.