@Lewis: so now Wm the Conqueror is "the infamous William the Conqueror."
Anyone who "thought it was all over with King Harold's death" knows practically nothing about the story of the Norman conquest of England.
A list (off the top of my head) of rebel factions and their allies:
Edwin and Morcar Alfgarson, erstwhile earls of Mercia and Northumbria. (Edwin was betrayed by his own men to death. Morcar probably died in prison after Wm's death in 1087.)
Waltheof earl of Huntingdon. The sole English earl left, married to Wm the Conqueror's niece, Judith. He goes over to the rebels (and loses his head for it).
Edgar the Atheling, the luckiest man alive in the late 11th and early 12th century (his sister, Margaret, marries Malcolm, king of Scots).
The aforementioned Hereward the Wake, operating out of the Fens, and the last rebel to go down.
Eadric the Wild, an Anglo-Welsh bandit, bent on plunder, not reestablishing the former order.
Bastard sons of king Harold, several of them, united with Welsh and Midlands supporters. They raided from there several times, and even around to the east coast, and had at their peak some 65 ships filled with men.
Malcolm, king of Scots, harboring the Alfgarsons and the atheling and numerous other refuges. Malcolm would like to see a "Scandinavian" kingdom of N. England, with his brother-in-law as king, beholden to Malcolm, of course.
Sweyn Estrithsen, king of Denmark, the most powerful force afloat. He invades England near the Fens with over 300 ships and an army comparable to that of Harald Hardrada. The Danes, however, are bent on plunder as Vikings and lack any concerted commitment to throwing down Wm the Conqueror.
None of these discontented opportunists cooperate well. Sometimes there are pitched battles, but the morale ascendency of the Normans and their English subjects (yes, most of England supported, actively, the new order) is the deciding factor again and again. Even in temporary defeat, King William's lieutenants never give up much less go over to the other side. That is the key or core factor in Wm's success in putting down the numerous rebellions and invasions. His troops are devoted to the consolidation of their new realm, including putting whole settlements and even towns to the sword if necessary.
Militarily, the "harrying of the North" was necessary. Morally it was excessive. But military concerns know of no moral limitations or considerations when the choices are defeat or survival. That's how closely run the putting down of the rebellions was for Wm the Conqueror. He almost lost. Without ruthlessly depopulating the whole of Yorkshire and the east coast, he would have allowed more rebellions to build there.
John Beeler's "Warfare in England 1066 to 1089" has quite a few details on these campaigns.
David C. Douglas' "William the Conqueror" is a good first read, around page 215 on.
Neither is a thorough or in depth study of the campaign. That book that Lewis pointed us to looks interesting….