In general, William of Apulia can be regarded as an accurate and truthful writer. Legendary and popular traditions are used sparingly in his work. The exclusive interest of the author is events in Apulia and the career of Robert Guiscard and his references to other areas needs to be substantially supplemented by the chronicles of Amatus of Montecassino and Geoffrey Malaterra. The work has many of the characteristics of the medieval gesta: its approach is fragmentary and selective; and, it is episodic. There is none of the chivalric writing that punctuates Malaterra or the visions and miracles of Amatus though it contains information lacking in other sources.
William writes generally good classical Latin; he was familiar with the principal Roman poets, and especially in his allusions to Vergil and Lucan, although direct textual references are relatively few. Both of these Roman authors were known in southern Italy during the Norman period. There were, for example, manuscripts in the libraries of the monasteries of Montecassino and Tremiti. William’s more contemporary sources are however difficult to identify. He was very well-informed about the Byzantine Empire, and there are similarities between his account of Robert’s attack on Byzantium and that contained in the later Alexiad by Anna Comnena that was completed in the 1140s. It is, however, unlikely that they both drew on a ‘lost common original’ source, as some historians have argued. William’s own work was not well known in the Middle Ages; the only later author known to have used it was Alexander, the author of the chronicle of the Abruzzi monastery of St. Bartholomew of Carpineto, writing in the last decade of the twelfth century. It was, however, also known in Normandy.
The poem was written in hexameters (epic verse), although this translation is in prose. Despite its title, the poem is not exclusively concerned with the life of Robert Guiscard, Duke of Apulia 1059-1085. The first book deals with the arrival of the Normans in southern Italy, and their early campaigns up to 1042. Robert first appears only in Book II, and the discussion of his career is very selective, concentrating on a relatively small number of heroic episodes. The poem, for example, passes almost immediately from his investiture as duke in August 1059 via a brief account of the rebellion against him in the autumn of 1067 to the beginning of the siege of Bari in August 1068. The last two books are devoted almost exclusively to Robert’s campaigns against the Byzantine in the Balkans in 1081-1085.