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The unpublished masterpiece is a common enough ghost at the academic feast, not least when the place is Oxford and the menu medieval history. For those who heard Karl Leyser's pungent and subtle lectures on medieval Germany in the 1950s and 1960s his was more to be regretted than most.

In his last 20 years Leyser became a steady, and then a prolific publisher of papers and lectures. These two volumes contain 24, of which seven are published for the first time, and four are translated from German by Timothy Reuter, who has also substantially restored several that were unfinished when Leyser died in 1992, supplied full notes and added an essay on Leyser as an historian which, true to his spirit, says a great deal in a short compass. The editing could hardly have been done better (though the copy-editing might) and it has been done in a very short time: it is a lasting monument.

"The great book" never appeared. Even the path-breaking Rule and Conflict in an Early Medieval Society: Ottonian Saxony (1979) is constructed and reads as a set of linked articles. But the two volumes of Communications and Power, like Leyser's first such collection, Medieval Germany and its Neighbours (1982), now an established classic, possess not only coherence but a breadth and unity of vision which are more than worthy of that description.

With two minor exceptions these papers date from the 1980s and early 1990s. Each one is self-contained and closely argued, but all contribute to a compelling account of the transformations of European society between the early decades of the 9th and the 13th centuries which ranks with those of the great masters of medieval history - of Pirenne or Bloch, Duby or Southern. Even the earliest piece here, a review from 1954 of a minor monograph on the German College of Electors - a good deal more minor when Leyser had finished with it - sets out perceptions and themes which remain central to his work.

For Leyser, history was a three-way dialogue between the actors, their historians and ourselves. His hallmark is the intense and sensitive interrogation of the great chroniclers of medieval Germany, led by Widukind of Corvie, Liudprand of Cremona, Thietmar of Merseburg and Otto of Freising. Often in the thick of the action, and always our guides and impresarios, they are never condescended to, or reduced to mere "sources". "The tension between the educated writings of the historians and the actual workings of society" is both a powerful source of illumination and itself a central thread in the pattern of historical change.

Leyser treated the chroniclers - much like his colleagues and pupils - with a sceptical but affectionate understanding of their limitations, enthusiastic appreciation of their insights and attainments, and shrewd comprehension of their view of the world. The combination saved him equally from the impoverished scepticism of "bias" and "inaccuracy" which was prevalent at the beginning of his career, when many still believed that "objective" history could be written from charters and public records, and from the jargon and excessive abstraction which has sometimes been the price of accepting that the possession of a formation and a point of view neither distinguishes a historian from a chancery clerk nor diminishes the value of his work.

Of one of his favourites, Thietmar, Leyser remarked that "more than any other writer of the Ottonian ambit (he) conveys a sense of place and occasion, the milieu of the ruling families and the parameters of their outlook and feelings". Those are just the gifts and preoccupations which gave Leyser's own discussion of the whole sweep of German and European history across five formative centuries such consistency and coherence.

The thickest clusters of papers form naturally around the Empire's two great periods of splendour and that of disaster which separated them: its formation under Henry the Fowler, Otto I and Otto II (and their queens, who figure prominently in two of the newly published papers), in the middle decades of the 10th century, and the reign of Frederick Barbarossa in the 12th century, separated by the cataclysmic conflict with the forces unleashed by the papacy of Gregory VII and his successors, and by the devastating Saxon rising of 1073 and the wars which followed it.

To meet these familiar themes in the context of a collection which opens with the evolution of the concept of Europe from classical times until the high middle ages and ends with debates on the morality of warfare from the Merovingian Franks to Marsilio of Padua, is to be forcefully reminded that the neglect of the German lands and their rulers over the past 50 years, and with it to a large extent the isolation of German historiography (for though we justly rebuke ourselves this has not been only a British disease) is not only indefensible in principle, but distorts in practice. Some of the most effective essays here show how familiar landmarks - the beginning of the Capetian dynasty in 987, the marriage of Henry I's daughter Matilda to Henry V - were actively shaped by German concerns traditionally thought peripheral. That is one example, if probably the most important, of the inclusiveness of Leyser's vision. His Europe grew out of the Carolingian world of the 9th century, but was no less the product of the violent upheavals of the 11th century, the "first European revolution" of one of the last lectures; in which lives and conflicts everywhere shaped by local pressures and the imperatives of family also responded to the sense of belonging to a wider community, becoming Latin Christendom, which the historians nourished.

His history combined learning with imagination, and cool realism with profound idealism as very few have done. It will be with us for a long time.

R. I. Moore is professor of medieval history, University of Newcastle upon Tyne.