William of Poitiers, The Life of William

 A volume containing the original text of The Gesta Willelmi, by William of Poitiers can be found on Project Gutenberg, in French. Accordingly, the material here quoted from the volume has been translated via Google and any translation errors arise there.
The same volume contains William of Jumieges (see previous post).

HISTOIRE DES NORMANDS, PAR GUILLAUME DE JUMIÈGE. —
VIE DE GUILLAUME-LE-CONQUÉRANT, PAR GUILLAUME DE POITIERS.
https://www.gutenberg.org/ebooks/64008

The original Latin text, edited by Giles in 1845,  can be found on Internet Archive.
https://archive.org/details/scriptoresrerum00gilegoog/page/n7/mode/2up

About The Gesta Willelmi from the preface by Giles.
“10. The next work is of a more important character,  being in fact the source from which we obtain most  of our information conceming the life of the first William. The Gesta Willelmi, by William of Poitiers, was first published by Duchesne, among the Scriptores rerum Normannicarum, from a MS. that had been lent  to him by Sir R. Cotton. This MS. was never retumed to its owner, and either perished in the French Revolution or still exists unnoticed on the shelves of some provincial library.
The work was again printed by Maseres in a volume entitled Historiae Anglicanau circa tempvs  conquestus Anglicae a Gulielmo Notho, Normannorum  duce, selecta monumenenta, Lond, 4to. 1807.
The whole  of that volume will be included in the present collection, except the Encomium Emmae, of which the English Historical Society are preparing an edition, and the extracts from Ordericus Vitalis, which it seems unnecessary to reprint, because a new edition of the whole of that author  will no doubt before long issue from the press."
[it begins on page 77]
https://archive.org/details/scriptoresrerum00gilegoog/page/n97/mode/2up?view=theater

Who was William of Poitiers?

Orderic Vitalis gives a biography of WOP in Book IV, chapter VII.
“Thus  far  William  of  Poitiers  carries  his  history,  which, imitating  the  style  of  Sallust,  eloquently  and  acutely  recounts the  acts  of  King  William.  This  author  was  by  birth  a Norman,  being  a  native  of  the  town  of  Preaux,  where  his sister  was  abbess  of  a  convent  of  nuns  dedicated  to  St. Leger.  He  is  called  William  of  Poitiers,  because  in  that city  he  drank  deeply  at  the  fountain  of  learning.  Returning into  his  own  country,  he  became  eminent  as  the  most learned  of  all  his  neighbours  and  fellow  students,  and  made himself  useful  to  Hugh  and  Gislebert,  bishops  of  Lisieux,  in ecclesiastical  affairs,  as  archdeacon  of  that  diocese.  He  had served  with  courage  in  a  military  career  before  he  took orders,  fighting  bravely  for  his  earthly  sovereign,  so  that  he was  the  better  able  to  describe  with  precision  the  scenes  of war,  from  having  himself  been  present  and  encountered their  perils.  As  age  came  on  he  devoted  himself  to  science and  prayer,  and  was  more  capable  of  composing  in  prose  or verse  than  of  preaching.  He  frequently  wrote  clever  and agreeable  poems,  adapted  for  recitation,  submitting  them without  jealousy  to  the  correction  of  his  juniors.  I  have briefly  followed,  in  many  parts,  his  narrative  of  King  William and  his  adherents  without  copying  all  he  has  written, or  attempting  to  imitate  his  elegant  style”.   
https://archive.org/details/ecclesiasticalhi02ordeuoft/page/46/mode/2up?q=poitiers

The introduction to the Volume states:-
“Guillaume de Poitiers is undoubtedly one of the most distinguished of our ancient historians; he lacks neither the sagacity to disentangle the moral causes of events and the character of the actors, nor the talent to depict them. He knew the Latin historians, and evidently endeavored to imitate them; also Orderic Vital and several of his contemporaries compared him to Sallust; he sometimes reproduces in fact, with enough success, the precision and the energy; but he falls much more often into affectation and obscurity. It is no less a great loss than that of the beginning and the end of his work; the first and last years of King William's life are absolutely missing in all the manuscripts. That of the Cottonian library, which is the most complete and on which Duchesne published his edition, begins in 1035 and ends in 1070."

William of Poitiers on Earl Godwin.
Following a description of the death of Alfred at the beginning of the text, William of Poitiers shows his lack of objectivity regarding Earl Godwin and his offspring.
“We therefore address you a short apostrophe, Godwin, whose name, after your death, survives you infamous and odious. If it were possible, we would like to scare you of the crime you have so wickedly committed. What execrable fury agitates you? With what heart could you meditate, against right and justice, such an abominable crime? Why, the most cruel of homicides, do you commit for the loss of you and yours the smallest betrayal? You congratulate yourself on having done what the laws and customs of the nations furthest removed from Christianity abhor; Alfred's outrages and evils excite your joy, O wickedest of men, and cause the tears of good people to flow. Such things are dismal to report.
But the most glorious Duke William, whose actions, supported by divine help, we will learn of in future ages, will strike with an avenging sword the throat of Harald, so similar to you in cruelty and perfidy. You shed by your treachery the innocent blood of the Normans; but in its turn the iron of the Normans will cause the blood of yours to flow. We would have preferred to bury this inhuman crime in perpetual silence; but we do not believe that even bad actions, necessary for the continuation of history, should be excluded from our writings, as we should forbid ourselves from imitation.”