Familiarity is a crucial element in narrative fiction reading for children, playing a significant role in social learning from storybooks. Nevertheless, distinct studies greatly vary in their interpretation of what renders a storybook familiar to a child, researchers' methods for measuring familiarity, and how researchers link familiarity to reading outcomes. This integrative review synthesized the principal ways in which researchers define and measure familiarity when investigating the impacts of storybooks on children's social learning outcomes. In particular, we derived four key definition categories of familiarity from the current empirical literature: personal relevance, anthropomorphism, social relevance and realism. We specified which outcomes are most impacted by which type of familiarity, in terms of broader outcomes (e.g. transfer of factual information from storybooks examined for anthropomorphism and realism) or specific learning effects (e.g. story comprehension with storybooks modified for personal relevance). Overall, our findings suggest that it is not familiarity per se, but rather its extent (how much the content of a storybook is familiar to the child prior to reading), that holds importance for social learning outcomes. The theoretical assumption of the distance hypothesis provides a compelling explanation for this finding.