Hannah Barker's That Most Precious Merchandise is one of the most important contributions to the historiography on the medieval slave trade. In particular, it provides a much-needed focus on the trade system that carried slaves from the Black Sea into the Mediterranean during the later Middle Ages. The strength of the book lies in the author's analysis of the three main importers of these slaves—Genoa, Venice, and the Mamluk sultanate—making it one of the few studies to successfully examine source material in Latin, Italian, and Arabic from these three perspectives. This includes not just printed sources, but a wealth of unpublished archival material from Genoa and Venice. By synthesizing these sources, Barker paints a picture of a complex and entangled trade system that was fully integrated into the commercial and political worlds of the three main players. In doing so she puts forward a fascinating argument that the Christians and Muslims of the Mediterranean shared a set of assumptions and practices that amounted to a “common culture” of slavery. To them, slavery was both legal and socially acceptable, and it was based on religious difference, partially articulated through linguistic and racial categories. Slavery was also considered to be a universal threat to all free people in the Mediterranean and Black Sea. This stimulating argument should provide scholars with food for thought for some time.

The book is split into two parts. The first examines how slavery was instituted in the late medieval Mediterranean and makes several assertions about the common culture of slavery, especially in relation to trade and the market. Here some fundamental assumptions about slavery in the period are explained, especially the thorny problem of the relationship between slavery and religion, which are tackled in Chapters 1 and 2. As Barker explains, although slavery was legally and socially acceptable to peoples of all religions in the Mediterranean, it was unacceptable to enslave those who shared your religion. In reality, this posed problems, such as the selling of Orthodox Christian slaves by Italian merchants, so language and race were used to categorize individuals who were enslaveable or not enslaveable in a particular society. This partly explains why, for example, Italians considered the enslavement of Russian and Bulgarian Christians to be more acceptable than Greeks (p. 48). Chapters 3 and 4 zoom in on the experiences of slaves, providing detailed examinations of the kinds of labor and service they performed, as well as the operation of the major slave markets in Genoa, Venice, Cairo, and Alexandria. Here Barker makes especially good use of statistical information by providing useful assessments of the demographic of slave populations and the changing prices of slaves. This is fleshed out with detailed consideration of factors such as gender, race, and the master's identity, and how they impacted the fate of slaves, especially for women who were frequently sexually exploited by their owners as well as being expected to carry out a range of domestic tasks.

The second part of the book studies various factors that impacted the Black Sea slave trade. It begins with an exploration of the chief mechanisms for enslaving free people around the Black Sea, especially violent capture and sale by relatives (Chapter 5), before focusing on the merchants who traded in slaves (Chapter 6). Here Barker convincingly argues that the slave trade was not conducted by professional slave traders, but by opportunist merchants, who bought and sold slaves alongside other commodities and transported them on mixed-cargo ships. The final chapter revisits the earlier discussion of the interplay between religion and slavery by considering the slave trade in the context of the papal trade embargo and crusade strategy on the one hand, and trade between Christian and Muslim states on the other. This allows Barker to rightly emphasize the diplomatic and religious consequences of the slave trade, alongside the purely economic.

The greatest strength of Barker's book is the synthesis of source material from the Genoese, Venetian, and Mamluk perspectives. Not only does this allow the author to present a nuanced depiction of an entangled trading system, but it also allows her to frequently debunk misconceptions surrounding various aspects of the medieval Mediterranean slave trade, especially when analyzed in isolation. For example, although it is widely accepted that women made up the majority of slaves in Italy, comparatively few scholars have shown that women also made up the majority of slaves in Mamluk societies, which were famed for their slave soldiers (pp. 69–70). Similarly, despite contemporary crusade theorists regularly decrying the role of “bad Christians” in helping ship slaves from the Black Sea to Egypt, evidence from Italian notaries and Mamluk commentators suggest that these major Christian slave traders, such as the famous Segurano Salvago, were in fact very uncommon (pp. 187, 197–98). Indeed, as Barker shows, very few merchants specialized in the slave trade in the way they did in the Atlantic during the 17th and 18th centuries and we know of no ships that were specifically designed for the transportation of slaves (pp. 152, 167–70). Thousands of slaves were still shipped from the Black Sea to Italy and the Mamluk sultanate each year, but this was facilitated by a vast web of individual merchants, agents, and brokers who rarely traded in large numbers of slaves at any one time.

One of the perennial problems facing scholars working on medieval slavery is the lack of sources that describe the life and experiences of slaves themselves. Barker attempts to overcome this obstacle by considering slavery in practice and thus digging below the official line provided by the normative sources. This is most successfully undertaken in Chapters 3 and 4, and although Barker does an admirable job of extracting information from the available material, there are some instances where I was left wondering whether the fascinating anecdotes could have been fleshed out with slightly more color in order to alleviate the density of the prose. For example, does the Venetian nobleman Giosafat Barbaro tell us any more about the episode where he witnessed the Mongols hunting and capturing a band of Circassians outside Tana (mentioned on p. 123)? Or are there any more details surrounding the grizzly murder of the Genoese slave Caterina (mentioned on p. 90)? But maybe both of these episodes, along with the many others dispersed throughout the book, are dead-ends where nothing more can be said. My only other minor quibble is partly semantic and regards Barker's concept of a “common culture” of slavery and whether this could have been explained with more precision, especially the definition of “culture” in this context. For example, although Barker convincingly shows how the Venetian, Genoese, and Mamluk slave trades shared common features, and thus came from one interwoven slave trading system (e.g., p. 211), I would suggest that there is a subtle difference between a shared system and a common culture which could have been explored in more depth.

Nevertheless, this is an impeccably researched and incredibly detailed study that brings together a wealth of published and unpublished source material from an impressive range of cultural and linguistic backgrounds. It is an excellent book and Barker should be congratulated on writing what should become regarded as one of the most important works on the medieval slave trade.