The study reported in this paper contributes to the literature on prospective teachers' decision making, specifically regarding how they analyze and make informed decisions about the use of their textbooks. We designed a training intervention with 28 future Costa Rican teachers, in which they had to identify and manage semiotic conflicts that can be found in a textbook lesson. For this purpose, we selected a lesson on proportionality from a textbook commonly used in Costa Rica. We performed an interpretive and exploratory content analysis of teachers' responses to the textbook lesson assessment tasks. The results show that participants progressed in identifying semiotic conflicts and could produce effective solutions for some of the detected semiotic conflicts. The prospective teachers reflected on the way they used the lesson by proposing improvements in more than one facet (epistemic, cognitive, affective, interactional, mediational, and ecological). We conclude that the training provided allowed the participants to progress in decision making regarding the use of the textbook lesson, although it is necessary to reinforce their didactic-mathematical knowledge on proportionality so that they can develop more critical and well-founded analyses.