William of Poitiers probably wrote his 'Gesta Guillelmi ducis Normannorum et regis Anglorum' (History of William the Conqueror) c.1073-4 (certainly after 1071 and before 1077). According to Orderic Vitalis
, who made use of his work, William was born at Préaux, Normandy, and, in his youth, was one of Duke William's knights. He gave up soldiering, however, to study at Poitiers (thus William of Poitiers). Returning to Normandy he became Duke William's chaplain, and then archdeacon of Lisieux. (William mentions that his bishop, Hugh, was alive at the time he was writing the 'History' - Hugh died in 1077). In fact there are no surviving manuscripts of the 'History'. André Duchesne published an edition in 1619, and the, now lost, manuscript he used was missing both the beginning and end. The work as it now exists covers the period from 1047 to 1068 - though it starts, mid-sentence (it also finishes mid-sentence), with retrospective material concerning affairs in England after Cnut's death (1035). Orderic Vitalis says that it originally finished in 1071.
The 'History' is, effectively, a panegyric to William the Conqueror, but, despite this, it is an extremely important source of material relating to the Norman invasion of England. In 'The Norman Conquest of England: Sources and Documents', R. Allen Brown writes:
"Within the panegyric there is a wealth of facts and details, some judiciously selected from other sources, not least William of Jumièges
, but most derived from personal knowledge and personal contacts, compiled and intelligently put together by a man uniquely qualified as both clerk and knight, closely connected with the court and, indeed, for years part of it. One may add that William of Poitiers must have known his hero from their joint youth up, and stress that as both former knight and former chaplain of the duke he is able to bring us closer to the heart of Normandy in the mid-eleventh century than any other writer of that age or later."