Both Heinz Werner and Lev Vygotsky present fundamental contributions to the comprehension of human psychological development, and their perspectives on the interaction between the cultural environment and the improvement of psychological functions dialogue fruitfully. When it comes to the development of conscious psychological functions, Werner’s orthogenetic principle of development can be interpreted according to, and interrelatedly to Vygotsky’s elaborations on the construction of spontaneous and scientific concepts. In this chapter, we propose an innovative concept of Additional Language Literacy Development (ALLD), based on those two fundamental conceptualizations, together with Jean-Paul Bronckart’s theory of language activity. The heuristic value of such a concept is explored by analyzing the transformations in verbal retrievals of reading in an AL made by one student immersed in a formal learning process of Portuguese language. The analyses of recollections of the student’s reading along the process demonstrate that the student, beyond the didactic mediation, built conscious resources to situate the texts into a genre and visualize their interactional context (author and purpose). These sociocultural resources can be understood as language scientific concepts that mediate the action of reading, and convert the interpretation process in the new language into a more differentiated, articulated, and hierarchically integrated activity. Finally, we discuss the contribution of the proposed concept to the interpretation of previous research outcomes in ALLD and the potentials of the new conceptual framework.