The basilica and episcopal complex built by Bishop Eufrasius at Parenzo (modern Poreč) on the Istrian coast in the mid-sixth century ad is one of the finest examples of early Byzantine art and architecture to survive into the modern age – as important as the churches at Ravenna, the monastic complex at Sinai and the Justinianic churches of Constantinople. It was a lavish product in imported Proconnesian marble with fabulous opus sectile marble revetments as well as spectacular figural mosaics. It has survived in surprisingly good condition.

A great debt is owed to Anne Terry and Henry Maguire. At last, one of the great churches of Justinian's reign – and its truly marvellous cycle of mosaics – has been properly published (this is really the first monograph since Bruno Molajoli's work of the 1940s) and wonderfully illustrated. The discussion consists of a painstaking series of descriptions (aided by excellent access granted to the authors during the recent restoration and excellent photographs taken from the scaffolding), a ground-breaking examination of the archives in Vienna of the late nineteenth-century restoration and hence the most up-to-date available judgements on what is authentic, how much of it precisely and what is pure fabrication by the nineteenth-century restorers. Frankly, as far as the authors' aims go, it is hard to be critical.

But, given the ambition and expense of this volume, what a disappointment it is that their aims were so limited. Poreč is exceptional because of the (not very good) survival of a series of mosaics on the exterior of the church – hugely important because so rare. These receive no discussion at all since the book is about the interior mosaics. Once Terry and Maguire have decided how much of the triumphal arch mosaics are nineteenth century, they illustrate them only with pre-restoration photographs and fail to give us any image of their modern state. Such decisions are reductive and intensely idealist – resisting the history of change and the current state of works of art as markers of their reception and experience. But they are also anti-scholarly because the pragmatics of expensive publications means that this may well be the last time in a generation that will see this material so well published (at any rate with wonderful plates) – and so, for what the authors have chosen not to discuss or illustrate, the bus has been missed. Further, because they choose to limit their discussion to the surviving original sixth-century mosaics in the interior, their interpretation of the images' meaning (in chapter vi, ‘Sixth-century iconology’) is fundamentally vitiated by not taking into account what we know of the basilica's full mosaic programme, inside and out – frankly, a very odd decision indeed.

Intellectually, their positivist interest in precisely what was restored means that they use the imperial archives in Vienna solely in pursuit of facts about authenticity rather than as a testament to ideologies of conservation. Yet the archives of the Imperial Royal Central Commission for the Study and Preservation of Art and Historical Monuments in Vienna offer potentially a far richer historical picture not only of the attitudes to and decisions on material intervention in the late Hapsburg empire, but also of the relations of the pragmatics of conservation with the theoretical work of some major scholarly innovators in the discipline of art history, and especially the art history of late antiquity. Terry and Maguire briefly mention Max Dvorak, one of the major figures of the Vienna School in the first two decades of the twentieth century who was a ‘Generalkonservator’, but they fail to mention the towering figure of Alois Riegl (general editor of the Central Commission's journal from 1902) who was not only the founding father of Viennese art history and especially of the study of late antique art but in his late work on the preservation of monuments a major theorist of conservation and a significant influence in the time (his 1903 paper ‘Der moderne Denkmalkultus, sein Wesen, seine Entstehung’ is the classic theoretical piece in its field). The point here is that the thousand pages recording all the debates about the Parenzo mosaics represents an extraordinarily intimate portrait in action of one of the most significant moments in modern times of the application of new ideas about conservation to a set of monuments newly discovered as centrally important, and especially so in Austro-Hungary which considered itself the direct continuation of the Christian empire whose artistic ethos the Parenzo mosaics were seen to encapsulate. But we get nothing of this in this volume. Despite the archival concern with the monument's later history, it is in fact solely in its original moment of creation that the authors are interested.

Within the purview of their interests, they handle the materials with the sureness of touch one would expect from so distinguished a team. But I worry about the decisions on artistic comparanda from the sixth century in matters of dating, facial types, iconography, mosaic laying, style and so forth, which systematically privilege the roughly contemporary churches and mosaics of Ravenna. The problem here is that this aligns Poreč with Italy (that is the sixth-century Byzantine expansion into the west) – which is not an empirical induction but an interpretative leap (especially in light of the relative paucity of comparable material surviving from the east). Indeed, as Terry and Maguire admit, while mason's marks at Parenzo are indeed replicated at San Vitale in Ravenna, very similar marks also appear in St Sophia in Constantinople. Yet Parenzo, on the very border line between east and west within the Byzantine empire, may in fact look both ways. Perhaps the extent of our knowledge is simply not enough ever to determine the answer finally, but it might be said that sixth-century mosaic apses with the enthroned Virgin are strikingly limited in Ravenna (only the lost apse of Sta Maria Maggiore is attested as a possibility) and more frequent if one looks east (three are known from Cyprus and others further east, such as Gaza, to which the authors allude on pp. 145–6).

I have done a lot of grumbling. In a way, this is itself a genuflection to the ambition of Terry and Maguire's project, the truly beautiful way in which it has seen publication and the outstanding excellence with which they have realised some parts of what I see as the big challenge of Parenzo.