This paper reports results from two studies conducted to examine word learning among preschool children in group book reading while we developed a scalable method of teaching words during book reading. We sought to identify factors that fostered both depth and breadth of learning by varying the type of information children heard about words while holding exposures constant. We also asked whether prior word knowledge affects children's learning across our different instructional approaches. In Study 1 we evaluated pre-post gains from two types of explicit instruction ("Didactic" and "Conceptual"), an implicit instructional approach ("Review"), and repeated "Exposure." For all three instructed conditions growth in receptive knowledge (our measure of breadth) was statistically equivalent when compared to control (d?=?0.43) and exposure words (d?=?0.41). In Study 2, words were taught using an augmented explicit approach and through repeated exposure. Moderate and statistically significant growth in receptive knowledge was found when comparing instructed to control words (d?=?0.48) and large effects were found with an expressive task measure of depth of knowledge (d?=?1.19). There also was evidence of learning from exposure. Children's vocabulary knowledge moderated learning gains. In Study 1 children with limited knowledge of vocabulary (0.75 SD below the group mean) learned fewer words than others. In Study 2, pre-test vocabulary knowledge moderated gains on the expressive measure for directly taught words and gains on the receptive measure for words taught through exposure. Thus, when words were intentionally taught, all children except those with the weakest initial knowledge acquired the initial lexical representations captured by the receptive measure at a similar rate. Those with stronger vocabulary more quickly acquired initial representations from exposure alone and deeper knowledge when they received intentional instruction. We conclude that teachers can build depth and breadth of vocabulary knowledge by combining intentional instruction of target words with repeated use of varied words by reading books multiple times and instructional comments that include use of novel words.