Book I covers the period from 1017 to 1043. It narrates the arrival of the Normans in Italy; their battles in northern Apulia under the command of Melus, leader of the rebellion against the Byzantine Empire in 1017; their defeat by the catepan Basil Boiannes in 1018; their settlement in the Campagnia between 1018 and 1038; their revolt in Sicily against Michael Dokianos where they served as mercenaries in the Byzantine army; their battles in Apulia against the Greeks in 1041-1042; the revolt of the Greek general George Maniakes and the resistance to this in Italy led by Argyrus aided by the Normans; and the death of Maniakes in 1043.
William of Apulia used contemporary historical works produced in Apulia especially the Annales Bariensis and the chronicles of the Anonymous of Bari and Lupus Protospatarius for the years 1017-1018 and 1041-1043. This may account for the detailed account of this period in the Gesta and slight discussion of the establishment of the first Norman settlements in and around Aversa. Perhaps, in a selective biography of Robert Guiscard, this is not surprising since William of Apulia’s focus was the de Hauteville family not the Drengots of Aversa and Capua. William of Apulia often copied the contemporary sources word for word but he did make some modifications especially to their chronologies: this can be seen in the error in the initial date of the invasion of Apulia in 1017 and the chronology of the operations of Maniakes in Apulia in 1042-1043. He also supplemented these sources with additional material: for example, the order of the Senate to behead Melus (I: 83); the presence of Armenians from Calabria who were Monophysites in the army of Dokianos (I: 334); the role of Gautier, son of Amicus at the battle of Montepeloso (I: 387); the election of Argyrus as leader of the Normans (I: 418) and the atrocities of Maniakes (I: 488, 499 and 569). He provided several precise pieces of topographical information like the site of Arenula (I: 66-73) and chronological details like the date of the battle of Cannae on the kalends of October (I: 92).
The author established an ‘Apulian’ tradition for the arrival of the Normans in southern Italy and parallels the views of Amatus of Montecassino of a ‘legend of invitation’ as a means of explaining the Norman presence there. It is possible to use William of Apulia to explain Melus’ presence on Monte Gargano: it appears likely that the area remained under Beneventan control until 1018 when the catepan Basil Boiannes fortified the Byzantine frontiers as far as Fortore. However, his discussion of the early Norman settlement in the Campagnia is brief and vague generally following popular traditions. Malaterra and Amatus are far more detailed and accurate on these two decades. The poet also omitted the part played by Guiamar IV in the partition of Apulia in 1042 or that he legitimated the authority of the new count William and there is a similar lacuna in his discussion of relations between the Normans and Adenulf of Benevento.
The degree to which William of Apulia used Byzantine sources for his discussion of the revolt of the Norman mercenaries in 1040-1041 and for the part played by Argyrus between 1041 and 1043 is a matter of some debate. Delarc suggested that there was a certain resemblance between the Gesta and the writings of John Skylitzes especially on the parts played by Michael Dokianos and the son of Basil Boiannes. However, it is not possible to establish a direct connection between the Greek sources and the Gesta. What is clear from the text is that the poet had a precise grasp of events in the Byzantine Empire during the reigns of Michael IV, Michael V and Constatine IX Monomarkos and that he consequently had access either to unspecified Greek sources or to an individual(s) with detailed knowledge of near-contemporary Byzantine history.
 Joranson, E., ‘The inception of the career of the Normans in Italy’, Speculum, vol. xxiii, (1948), p. 353 argued that Monte Gargano was under Greek control at this time and that it would have been impossible for Melus, an exile from Bari to have stayed there between 1010 and 1016.
 Delarc, O., Les Normands en Italie, Paris, 1883, p. 110, no 1.