There is robust evidence that reading aloud with young children can help them learn new vocabulary. Building upon prior research, this study tested the effects of "both" book text features "and" readers' spontaneous extra-textual word-highlighting strategies on 3- to 4-year-olds' vocabulary retention from repeated read alouds of a story in their own homes. Parent-child dyads (n = 30) were provided with either a rhymed or unrhymed version of the same story featuring novel names and illustrations of eight imaginary monsters, along with an audio recorder to take home. Parents recorded reading the book aloud with their child daily for five consecutive days, then children were tested on their monster name recall. Recordings of each dyad's five progressive read alouds were transcribed, and coded for conversational elements and novel word-highlighting strategies. Findings indicate that children retained the novel monster names with equal success from rhymed and unrhymed books. While individual variation among dyads in their extra-textual commentary was high, each tended to adopt a consistent amount while reading the book across days in both rhymed and unrhymed formats, even as they became more familiar. The one extra-textual vocabulary highlighting behavior during readings that was most predictive of children's later monster name recall was their own frequency of making guesses of the monster names before they were heard during the read alouds. A close look at the interactions between book format, book familiarity, and the extra-textual commentary around a storybook, and the implications of these interactions for vocabulary building are discussed.