This article quantifies books' impact in terms of citation counts - in published articles and in other published books. The average political science book published by a university press receives about three times the number of citations received by an article indexed in the SSCI. Books' impact varies by subfield, with books published in methodology receiving many more citations on average than books published in other subfields, followed by books in international relations. Overall, books published on American politics are cited least frequently. Results suggest that political scientists should supplement quantitative indicators of article and journal impact (which are based only on citations in peer-reviewed articles) with similar measures that account for the scholarly influence of published books.