When was it written?
This poem, one of our main contemporary sources for the Norman Conquest of Italy, was composed between 1096 and 1099. The dedication in the prologue to Urban II gives a terminus ante quem, for the pope died on 29th July 1099 and a reference, in Book III, to ‘the Gallic race [who] wanted to open the roads to the Holy Sepulchre’ suggests that William was writing after the beginning of the First Crusade, called by Urban II in November 1095 at Clermont. The influence of Roger Borsa (1085-1111) to whom the poem is also dedicated is evident in those passages where William of Apulia established the legitimacy of this prince especially his conflict with his half-brother Bohemond following the death of their father in mid-1085. William declared that the power of Robert Guiscard was founded on his military ability but had been legitimated by his marriage to Sichelgaita of the ruling Lombard dynasty of Salerno and the papal investiture at Melfi in 1059. The ‘selective’ nature of Guiscard’s biography can be seen as a legitimating document for the Hauteville family in Apulia and Calabria.
Who was William of Apulia?
Nothing is known of the author except his name. There is nothing in his poem about his birth or family background, about which Geoffrey Malaterra provides information in his contemporary prose biography Deeds of Count Roger of Sicily. There has been a debate, extending over the last three centuries about whether William was a Norman or not; it is inconclusive and there is nothing in the Gesta to prove that the author was a Norman any more than he was an Italian. William was an unknown name in southern Italy before the Normans but this proves little: he could have been a Norman called William or equally a Frenchman and there is no reason why a Lombard family could not have called their son William in the aftermath of the Norman Conquest. His relationship with the French pope Urban II seems to suggest that he was French, though not necessarily from Normandy. However, it is plausible that, like Bohemond, he was ‘loco Appulus, gente Normannus’, ‘born in Italy of the Norman race’ or, like the Norman monk Malaterra ‘noviter Apuliam factum’, ‘newly come to Apulia’.  His knowledge of Apulia, its history, geography, the annals of the province and his general interest in it is seems to justify the epithet ‘of Apulia’.
His treatment of Lombards is distinctly more positive than the images that permeate the histories of Amatus and Geoffrey Malaterra. Whether this means that he ‘had Lombard blood in his veins’ as Wolf strongly suggests is far less certain. Should we regard the Gesta like Amatus’ Historia Normannorum as history ‘written from the other side’? Certainly for Amatus as a Casinese monk, this meant transforming Norman aggression into the kind of defensive military activity one would expect of ‘protectors’ whose task it was to promote the security of Montecassino and Christendom as a whole. If William was a Lombard, this meant finding a place for the Normans within a Lombard historiographical tradition that extended back to the Historia Longobardorum of Paul the Deacon in the late eighth century. The emphasis William places on the transition of the Normans from people without roots into ‘stabiles’ through the series of marriage alliances that linked the sons of Tancred to Guaimir of Salerno is fundamental as he explicitly linked the rise of the Normans in Italy to their incorporation within the existing Lombard political structure. This allows him to treat the deeds of Robert Guiscard as a continuation of the deeds of the Lombard princes of southern Italy, the culmination of five centuries of struggle to remove Italy from Byzantine control. It may also help to explain the contrast between the triumphant approach of the first three books when the Normans are fulfilling a historic mission to expel the Greek interlopers from Italy and the darker final books where Guiscard perverts that mission with his expeditions into the Balkans.
There is a lack of reference to the Scriptures in his work though his devotion to Urban II, St Matthew and Gregory VII comes across strongly and this has generated a debate over whether William was a cleric or a layman. William did not try to shape Robert Guiscard into a defender of the church as Amatus had done. Although there are occasional references to divine intervention especially in his descriptions of Civitate and during the siege of Durazzo, the only real exception to William’s secular tone was his account of the Norman campaigns in Sicily.
William was probably a member of Roger Borsa’s court. This would explain the Gesta’s close attention to Guiscard’s designation of Roger as his heir on the eve of the first Balkan campaign. Indeed, it is possible that Roger commissioned the work as a means of solidifying his claim to Apulia in the face of the ever-present challenge posed by his half-brother Bohemond. At the very least, like Dudo of St Quentin almost a century earlier, William is a Norman historian writing Norman history at a Norman court for a Norman prince. It is this, not his origins that determined the nature of his writing.
 Some historians have suggested that because he is critical of the Normans and especially their cupidity that he could not have been a Norman himself. However, when instructing Geoffrey Malaterra on his biography, Roger of Sicily made it clear that he wanted Malaterra to tell the story ‘warts and all’. It is perfectly plausible that William of Apulia had the same instructions from Roger Borsa.
 William of Apulia’s impartiality is, however, remarkable in a medieval writer. Most wore their objectivity very lightly and wrote for a specific purpose. However, it is clear that William’s primary aim is to establish the legitimacy of Roger Borsa and it is not impossible that Roger could have chosen a Lombard to sing the praises of his father. Italian writers would have been equal to the task and the cultural renaissance at Montecassino and the poetic traditions of Salerno were evident in the literary output of Apulia in the last decades of the eleventh century.
 A certain ‘Willemus Apulus’ in 1092 went from Bordeaux in the company of other clerks to arbitrate in a dispute between the abbey of la Trinité de Vendôme and St Aubin d’ Angers. It may be that this is the same individual as ‘Guillelmus Apulus’, monk of Marmoutier who acted as judge in a dispute between the abbey of St Nicholas and St Aubin d’Angers in 1098. Identifying him with William of Apulia is possible since Urban II had close links with the monastery of Marmoutier from which the monk Ranger became archbishop of Reggio in Calabria in 1092. The problem with this identification is that at the critical time (1092-1098) when it is likely the Gesta was written ‘Willelmus’ or ‘Guillelmus Apulus’ is attested as being in France.
 Ibid, Wolf, K.B., Making History: The Normans and their Historians in Eleventh-Century Italy, p. 127.