Specialist scholarly books, the key form of publication in the Humanities and some Social Sciences, are often the forgotten younger-siblings of scholarly big data projects. Humanities disciplines are 'peer review' rather than citation-based. Demand for data on how outputs perform - from researchers, institutions and funding agencies - has been lower than in the case of the exact sciences. Like book publishing more generally, large-scale and timely data about distribution and usage of monographs has been hard to collect (or to access) until recently. No single publisher controls more than 4% of the global market. Books have been slower than journals to transition to digital formats[1]. Uniform approaches to DOIs and metadata are taking time to crystallise. Users engage with books in ways that are often difficult to 'see' at scale. Books have a much longer shelf-life than do articles; their impact can be measured in years and sometimes decades [2]. In this talk I present an applied, deep-dive case study of efforts to identify and track the uses of Open Access (OA) specialist scholarly books at global scale, through the lens of the Knowledge Unlatched project [3]. Knowledge Unlatched Research is engaging with the data generated by the larger Knowledge Unlatched OA monograph initiative to help libraries, authors, publishers and research funders to understand what happens to their books once they have been made OA; and to explore what changes (and what doesn't) when open access licenses are applied to long form publications. I show that rather than being a disadvantage, many of the factors that make data about books difficult to gather at scale are actually strengths. A diverse publishing landscape, a high-proportion of mission driven publishers, and libraries that care deeply about books and the communities that engage with them, are powerful advantages for HSS researchers. Helping communities to tackle challenges of coordination, transparency and trust as they relate to data about scholarly books, rather than simply going after 'low hanging fruit', will be vital to ensuring that big-data innovations help rather than harm the Humanities. However, there are signs that this is happening. Because diverse smaller stakeholders hold the data that relates to specialist scholarly books, there are structural incentives for libraries, publishers, research-funders and researchers to work together to make conscious decisions about how data are aggregated, and by whom. Questions arise about whether this is best tackled within commercial or public spaces, in light of what is now known about the value of being able to look across datasets to tell rich stories about engagement and impact; as well as the dangers of oversimplification of research metrics and the commercialisation of data resources [4]. It may yet turn out that being the forgotten younger sibling will allow a viable alternative system to emerge.