This chapter compares the holdings of scientific books in two private libraries of the 12th/18th century, one owned by a family of Muslim scholars in Damascus and the other founded and developed by a family of Christian physicians in Aleppo. It reflects on the overlap, similarities and differences between the two collections. In doing so, this chapter introduces a history of the book approach to the study of scientific literature, one that is not mainly concerned with ideal audiences and the intentions of authors but also with discovering the actual people with a documented interest in specific works.