Could the work of a heretic be useful? As Hannah Marcus emphasises in this fascinating book, early modern Italians repeatedly answered this question with a resounding affirmative. Taking medical knowledge as her case study, Marcus provides an illuminating account of the complexities entailed in Counter-Reformation censorship of ‘useful’ books. Following the introduction of Paul iv's Index librorum prohibitorum (1559), physicians were prominent amongst those who sought to retain access to prohibited books of professional relevance. In response, the Church turned to the practice of expurgation – that is to say, removing some portions of text while leaving the rest intact – to ‘purify’ books while keeping them in use.

The story that unfolds is one of contestation, negotiation and frustrated expectations. Marcus explores a largely futile attempt to enlist a committee of (predominantly lay) experts to compile lists of expurgations at the University of Padua, a celebrated centre of early modern medical knowledge. The scheme ran into numerous problems, including a lack of coordination between the centre and regional hubs. Some individuals co-opted the process for their own ends: the philosopher Cesare Cremonini, for instance, seems to have viewed involvement in the process of medical censorship as a chance ingratiate himself with the authorities. As Marcus notes, this ‘strategic choice’ paid dividends when Cremonini was himself investigated for heterodoxy.

Subsequent chapters focus on the more successful (though far from comprehensive) Index expurgatorius of 1607, a printed volume containing lists of official expurgations. Enacting these changes in one's books became a standard condition of reading licences. Marcus memorably describes expurgation as ‘the dark side of commonplacing’ (p. 94); its practitioners reversed the humanist practice of noting edifying passages to achieve new, censorious ends. Through the papers of Girolamo Rossi, a physician from provincial Ravenna and particularly dedicated lay expurgator, Marcus highlights yet more inefficiencies in the system: while some of Rossi's expurgations were printed in 1607, many were not, leaving no mark beyond the archive.

From surviving expurgated books themselves, Marcus draws out evidence of a wide range of censorship practices, ranging from the permanent (book-burning, precise removal with a blade, and thick indelible ink marks) to the more half-hearted (thin crossings-out and the insertion of blank pieces of paper, which could be filled back in). She also explores how libraries responded to these constraints, probing tensions between the competing ideals of the Bibliotheca universalis and the Bibliotheca selecta. Various compromises were found: in the Biblioteca Marciana, for instance, prohibited books were catalogued as ‘diversi’ – a miscellaneous and disordered grouping which meant they would not be happened upon by accident.

Practice repeatedly undermined theory, and as time went by censorship became increasingly symbolic. The practice of removing a heretical author's name was not, Marcus suggests, actually meant to wipe the name from readers’ memories; instead, it was valued as a means to delineate the boundaries of the Catholic community. An interest in policing confessional boundaries was shared by many involved in the censorship process, fitting into broader narratives of Counter-Reformation confessionalisation. The discourse of ‘utility’ gave Catholics a framework within which they could continue to exploit non-Catholic expertise, while simultaneously flaunting their own piety by crossing out – or at least adding disapproving labels to – the names of their ‘lapsed’ colleagues.

This is book history at its most human. In Marcus’ hands, minor annotations bring the readers of the past to life. The images scattered throughout this beautifully produced book make clear the processes described in the text and evince a broader material context. A painting of father and son Girolamo and Stefano Coli, sitting proudly amongst their books, is a touching addition to Marcus’ analysis of their requests for reading licences. She never strays too far from the physical and the bodily, noting that bored or tired censors frequently abandoned their efforts, while readers could always be relied on to behave in unexpected ways. One Paduan book owner claimed he had not realised a certain work was prohibited, having simply found a copy in ‘the place where he went to urinate’.

Marcus handles her dazzling array of manuscripts with virtuoso skill, operating across multiple languages and an enviable spread of regional archives. The detailed treatment of Rossi's papers is particularly impressive and suggestive, with Marcus using Rossi's expurgations to illustrate how the process of censorship helped him to shape his own distinctive understanding of scholarly piety. This is a rare insight into the way in which an early modern lay professional in Italy approached his religion.

Occasionally, Marcus might have pushed her textual analysis further. For instance, she frequently mentions Antonio Possevino's Bibliotheca selecta (1593) as an example of the more restrictive understandings of censorship advocated in Counter-Reformation polemic. Yet Possevino's detailed discussion of the ideal medical library in book xiv of that work is neither mentioned nor cited, despite dealing with intriguing questions such as the validity of Paracelsian chemical medicine. Marcus’ story is one in which the practical considerations of censorship out-run ideal theories; none the less, these ideal accounts might repay closer attention. Were censors aware that they were deviating from these ideals? Or were the ideals themselves also more flexible than we might assume, even in the apparently extreme case of Possevino?

Marcus ends with a reconsideration of the Galilean controversy. Like the physicians surveyed earlier in the book, Galileo turned to ‘utility’ as a defence of his (and Copernicus’) work. Marcus suggests that Galileo was building on defences of medical and other professional texts developed during the preceding century. She is surely right: advocates of expurgation had inadvertently supplied authors with a set of arguments which could also be marshalled in support of controversial new works. Marcus ends on a triumphant note: ‘Although Galileo was condemned, the justifications of utility and expertise that he advocated were the winning discourses of scientific rhetoric in the seventeenth and eighteenth centuries.’ But I wonder if ‘winning discourses’ is a little strong. Though utility was a winning discourse, it would rely on being closely allied to a Catholic conception of truth for a long time yet.