They sung how in the Conqueror's fleet Lord William ship'd his powers,
And gain’d a fair young Saxon bride, With all her lands and towers.
Ex Registro Monasterii de Whitby—In the Harleyan Collection, No. 692 (26) fo. 235. William Lord Percy, the fyrst founder of Whitby, his armes “Field azure, 5 millpikes 0r.”
He begat of Emme of the Porte Lady Percy,
Alayne Percy, who by Emme of Gawnt his wife begat William that succeeded him, Walter, Jeffrey, Henry, and Alayne - and he lyeth buryed in the Chapter House of Whitbye,
and his mother Emme of the Porte.
Which Emme fyrst was Lady of Semer besides Skarburgh afore the Conquest,
and of other landes, William Conqueror gave to Sir William Percy for hys good service;
and he wedded hyr that was very heire to them in discharging of his conscience.
Gul. Percy dedit. Whitby in opus restaurandi monasterii ubi Stephanas Abbas suit.
Idem Gul. Percy Whitby monachis abstulit.
Stephanas Abbas caepit cum suis restaurare monaster S. Mariae Ebor. favente ac juvante Alano comite Richemondiae, A. D. I088.
Ex Libello Stephani li Abbatis S. Mariae Eboraci. Lel. Col. v. 1. p. 22.
https://books.google.com/books?id=dhsvAAAAMAAJ&pg=PA221&lpg...
"A View of Northumberland: With an Excursion to the Abbey of Mailross in Scotland, Vol. II" by William Hutchinson (1776); pp.220-1 (including footnotes).
Since William Percy's wife was Saxon, she could not have been a random member of the Normandy, France, de Port family. Furthermore, we haven't seen any primary or even secondary sources to prove otherwise.
https://books.google.com/books?id=BFGDIde6L-wC&pg=PA60
https://www.indiebound.org/book/9780521524650
I'd be interested in reading or hearing what Judith Green might say on the subject. Her books are published by Cambridge University, and she has written extensively on the subject of Early Norman England.
https://en.wikipedia.org/wiki/Judith_Green_(historian)
"...an English medieval historian, who is Emeritus Professor of Medieval History at the University of Edinburgh. A graduate of King's College, London and Somerville College, Oxford, she held a research fellowship and then a lectureship at the University of St Andrews before transferring to a lectureship at Queen's University, Belfast. There she became a Reader and, eventually, Professor. In 2005, she took the professorship at Edinburgh, retiring in 2011."
Specialising in Anglo-Norman England, her notable works include:
The Government of England Under Henry I, (Cambridge, 1986)
The Aristocracy of Norman England, (Cambridge, 1997)
Henry I, King of England and Duke of Normandy, (Cambridge, 2006)
https://books.google.com/books/about/Forging_the_Kingdom.html?id=2q...
Erica Howton, I feel very strongly that the hard, primary/secondary source evidence weighs heavily in favor of Emma being a Saxon. The book you cited was very vague about her origins, and apparently without citing any source to reference for their claim of her being a member of the Norman de Port family.
I have only recommended Professor Green's books here (because I think they would be great reading material for the subject at hand), but I'm certain the first book I mentioned above provides sufficient corroborative reference in the form of the transcribed records from Whitby which state that Emma was Lady of Semer and Skarburgh before the Conquest and before she married William de Percy. Likewise, the minstrels identified her as a Saxon in their biographical verses.