Orderic Vitalis gives a short biography of him in his Historia ecclesiastica and says that he also wrote verses.
He wrote a eulogistic life of the duke, Gesta Guillelmi II ducis Normannorum, the earlier and concluding parts of which are lost; the extant part covers the period between 1047 and 1068 and is valuable for details of the Conqueror's life, although untrustworthy with regard to affairs in England. It gives a detailed description of the preparations for the Norman Conquest of England, the Battle of Hastings and its aftermath. According to E. A. Freeman, "the work is disfigured by his constant spirit of violent partisanship." It was written between 1071 and 1077, and was used by Orderic Vitalis.
References
There is a translation of the Gesta Guillelmi into modern English: The Gesta Guillelmi of William of Poitiers (ISBN 0-19-820553-8).