Books IV and V recount the last years of Guiscard’s life (1078-1085) and are far darker in tone than the opening three books. They contain important details of events. Book IV begins with the marriage of two of Guiscard’s daughters to Ebles de Roucy and Raymond Berenger II of Barcelona (IV: 8-15), his alliance with Pope Gregory VII against Henry IV with an original description of the reconciliation between Guiscard and Gregory in 1080 (IV: 16-73). The remainder of this book deals with the war between Guiscard and the Byzantine Empire on the pretest of restoring to the throne Michael VII and his young son Constantine betrothed to Guiscard’s daughter Helena, deposed by Nicephorus Botaniates. This is interrupted by the return to Guiscard to Italy in 1082 to deal with a new revolt (IV: 524-535) and his taking of Rome where Gregory VII was besieged by Henry IV (IV: 536-570).
Malaterra briefly describes the war though Anna Comnena’s account in The Alexiad is more detailed. However, William of Apulia’s version is preferable to Anna Comnena in several respects and helps to rectify her errors especially her belief in the involvement of Guiscard’s son Roger Borsa in the first part of the expedition and her chronology of events. William presents the most plausible sequence of events.