"How was William able to maintain control of England following his victory in 1066?"

Three major topics you'll be wanting to cover are: demographic change in the English nobility and the restructuring of land tenure, the physical imposition of Norman control on the landscape by means of castles (and to a lesser extent Cathedrals), and direct military action against rebels, rivals and dissenters.

For the first area, Domesday Book will be a really useful resource. It can be found online via the Open Domesday project run from the University of Hull, but it's always nice to sit with a physical copy. Helpfully, Domesday Book usually lists who owned any stretch of property in 1066 and who owns it now (as in 1086), and it's a really useful way to track how the Anglo-Saxon and Anglo-Scandinavian elites were almost entirely replaced by Norman nobility.

Pastscape and Heritage Gateway are useful resources for finding the Historic Environment Records that each council keeps from archaeological investigations of heritage sites in their jurisdictions and can be a real treasure trove of information about Norman castles. You should be able to cite HERs as essentially primary sources.

In terms of contemporary or thereabouts literary sources, look at Orderic Vitalis, William of Malmesbury, William of Jumiege's Gesta Normannorum Ducum, the Chronicon ex Chronicis of Florence of Worcester, Henry of Huntingdon's Historia Anglorum, and Symeon of Durham's Libellus de Exordio. The Anglo-Saxon Chronicle will be useful to you as well, and for a useful outside perspective, the Annales Cambriae and the Brut Y Tywysogion are very useful Welsh sources from which we can see the means by which the Normans attempt to extend control over Wales as well as England.

In terms of secondary reading, I always recommend Strickland's Anglo-Norman Warfare, Davies The Age of Conquest and Domination and Conquest and Christie, Creighton and Edgeworth's Transforming Townscapes: From Burh to Borough: the Archaeology of Wallingford, AD 800-1400 for a site specific case study of the physical imposition of Norman control.

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