Book III covers the decade between 1068 and 1078. William of Apulia interrupts his narrative of the siege of Bari with a resumé of the reign of the Byzantine emperor Romanus IV Diogenes and the battle of Manziket in 1071. His account shows a detailed reading of contemporary Byzantine sources. He includes details of the role of the Norman Joscelyn in the capture of Romanus by the ambassador of John Ducas and of the alliance between Romanus and the Turks and Armenians (III: 94-99). The narrative then returns to the siege of Bari and its eventual surrender in 1071 (III: 111-162).
In his account of the expedition to Sicily and the siege of Palmermo in 1073 (III: 163-343), William of Apulia provides detail of the fighting (III: 215-295), the participation of prisoners from Bari (III: 163, 187 and 322), the attempt by a Muslim fleet from Africa to assist the Arabs in Sicily (III: 225-228) and the installation of a Norman emir in Palermo (III: 342). What is neglected is the role played by count Roger, Guiscard’s brother in this campaign and his central role in the taking of the city, something that Geoffrey Malaterra considers in far more detail.
The Gesta is a crucial source for the rebellion by Normans against Guiscard in 1073 and especially 1078 (III: 354-411). Neither Amatus nor Malaterra describe events in Calabria or the rebellion in 1071 during the siege of Palermo. William provides several possible reasons for discontent against Guiscard’s rule especially the position of Richard, the young son of Geoffrey of Tarento that led to the revolt of Peter II of Trani in 1073. For the 1078 rebellion, William is the principal source especially as Amatus’ narrative ends in May 1078 and Malaterra makes only a few comments on Calabrian affairs (III: 509-687). He alone mentions that Baldwin and Henry de Monte Santa’Angelo were among the conspirators who rebelled against Roger Borsa and that Abelard went into exile in Greece under Alexius Comnenus. He deals with the duplicity of Argyritzos who first helped Guiscard capture Bari in 1071 only to betray him and deliver it to Abelard. His account of the siege of Salerno in 1076 roughly parallels that of Amatus though it is likely that the two sources were independent of each other.
 Mathieu, M., ‘Une source négligée de la batille de Mantzikert: Les Gesta Roberti Wiscardi de Guillaume d’Apulia’, Byzantion, vol. xx, (1950), pp. 89-103 considers this issue.