In the context of the critical need to support children's early language development, teacher knowledge may enhance children's opportunities to build linguistic skills. In this study we explored how early childhood teachers' (n = 86) pedagogical content knowledge for language and vocabulary, and their book-reading implementation across the school year independently and jointly predicted children's (n = 582; mean age = 49.76 months, SD = 7.06) growth and spring status on five standardized measures of vocabulary and syntax. Results indicated modest book-reading durations, on average, but also variability across teachers. Whereas there were limited or no main effects for book reading or teacher knowledge there were significant moderation effects in 6 of 10 models when predicting spring status and in 5 of 10 models when predicting growth. Findings suggest that longer fall book readings may be especially beneficial when teachers have low pedagogical knowledge, but that this pattern does not apply later in the school year. We discuss implications for future research, for understanding the constructs of knowledge and their role in authentic classroom practices and for professional development. [This manuscript was published in "Reading and Writing" v35 n8 p1815-1838 2022.]