Researchers suggest that exposure to greater lexical diversity provides toddlers and preschoolers better language outcomes, while shared book reading is known to facilitate the outcomes. This study examined whether reading to infants played a similar role and whether maternal lexical diversity during infancy is crucial for subsequent language development. Maternal language input at 6 months was videotaped during book reading and toy play and transcribed for analysis. Children’s expressive vocabulary was measured at 18 months. Results found that mothers used more diverse vocabulary and spoke more words with their 6-month-old infants during book reading compared to toy play interactions, with maternal lexical diversity at 6 months significantly predicting children’s expressive vocabulary at 18 months after maternal education and infants’ gestation week were controlled. These findings highlight the value of lexical diversity to preverbal infants while shared book reading is an effective way to boost the richness of parental language input.