A study explored the storybook reading experiences between Ms. Garza and her children. A broad conception of the zone of proximal development, involving use, adaptation, and transformation of culturally shaped tools in the process of shared activity, provides the framework for examining this particular Mexican-American family's reading behavior. Data resulted from taping and transcribing storybook reading events during two periods, approximately 2 weeks in length, in the home. The Garzas represent many of the Latino immigrant families served by southern California urban public schools: low income; primarily Spanish-speaking parents; and children in a bilingual school program. The family actively supports their three elementary-age children's education. At the beginning of each taping period, the family was provided with various children's books in Spanish, from which they could choose freely. Several books related to their cultural heritage were included before the second taping. Grounded categories were developed representing the language functions in the first taping, and after transcription and coding data from the second taping, these grounded categories were confirmed and added to. Four categories of book reading interaction were elaborated: reading practice; adult-directed exchange; collaborative interpretation; and cultural transmission. The variety of interactions defies generalization. Findings reveal the key influence of content on the family's book-reading interaction. Data also reveal a process in which an immigrant family appropriates a tool from the host culture, infusing it with their own purposes and forms. The cultural activity which the Garzas engaged in during storybook reading suggests the need to locate shared activity within their sociocultural environment. Family literacy programs for diverse communities should be sensitive to families' sociocultural realities. (Contains 25 references, and "Categories of Language Functions" is appended.) (NKA)