This paper briefly reviews Brazilian textbook policies during the twentieth century, and pays careful attention to its latest development, PNLD--"Programa Nacional do Livro Didático" (National Textbook Program) and its textbook assessments, which selects textbooks that are freely distributed by the Ministry of Education (MEC) to students of public elementary, middle and high schools. We focus on the mathematics textbook assessments, describing their evolution and commenting on some of their accomplishments and problems. The first assessment was carried out for the 1997 school year and retained basically its initial formulation, with changes and improvements, til the assessment for the 2018 school year. As of now, the program is undergoing substantial changes which are briefly discussed and which worry many educators. This assessment was responsible for a definite improvement of mathematics textbooks for elementary, middle and high schools in Brazil. It constitutes a good example of a successful program, coordinated by competent persons from mathematics education, schools of education and mathematics departments of good universities, at first without political interference, and strongly backed by powerful officials in MEC. In the last few years, pressures from publishers and conservative groups have undermined the assessments, with possibly serious consequences.