This study investigated the influence of book genre (narrative or didactic) on mothers' language use during a book sharing interaction with their 18- to 25-month-olds. Mother-child dyads were videotaped sharing both a narrative and a didactic book, adapted from two commercially available books, and matched in terms of length, quantity of text, and target content. A greater proportion of mothers' talk was complex (i.e., predictions, text-to-life comparisons) during narrative book sharing than during didactic book sharing. Mothers also used a greater variety of verb tenses and referenced more mental states during narrative book sharing. These results differ from findings from previous studies with older children where it has been concluded that didactic books offer greater opportunities for complex talk than narrative books. The results also highlight the importance of taking situational factors into account when investigating parent--child communicative interactions.