This study examined the ways in which the language that Head Start teachers used during book reading, as well as the extent to which they made explicit connections between book reading and other instructional activities, were linked to preschoolers' vocabulary development. Participants included 10 Head Start teachers and 153 children in their classrooms. Research Findings: Analyses revealed that teachers varied substantially in the frequency and nature of their book-related remarks, and connections between books and the broader curriculum were relatively few in number and constrained in nature. On average, children learned more words over the course of the year when teachers used more contextualized and decontextualized talk during book readings. Contextualized book-related talk was most positively associated with learning among children with relatively low initial vocabulary knowledge. Too few connections between book reading and the curriculum were observed to afford analysis of their contributions to children's vocabulary skills. Practice or Policy: The findings show the nuanced ways in which shared book reading, a critical part of the preschool day, is linked to vocabulary growth among the nation's most vulnerable learners. The results also highlight potential avenues through which readings could be strategically individualized to optimize early vocabulary development. (Contains 3 tables and 2 figures.)