Research Findings: This study examined the association between interactive book reading quality and prekindergarten children's gains in language and literacy skills over the course of an academic year for 96 teachers and 417 children across multiple locations in the United States. Two moderators were examined, namely, children's initial skill level and teachers' classroom organization during an interactive read-aloud session. In keeping with prior research, interactive book reading quality, measured by examination of teachers' extratextual talk, was positively and significantly related to children's development of both language and literacy skills. Interactive book reading quality was not significantly moderated by children's initial skills, with the exception that interactive book reading quality appeared most beneficial to children's print knowledge for children entering prekindergarten with relatively low skill. The unique contribution of this study is that the association between interactive book reading quality and children's expressive vocabulary was dependent on an organized classroom during the book reading session. Practice or Policy: The quality of teachers' talk during interactive book reading may matter for children's vocabulary development only when reading sessions are characterized by relatively high classroom organization. Classroom organization should be an important consideration while planning for interactive book reading sessions.