Peter Coss, The knight in medieval England (1993), examines one group among the minor landowners in a long perspective. Sally P.J. Harvey 'The knight and the knight's fee in England', Past and Present 49 (1970), pages 1-43, provides the fundamental analysis of the Domesday miles and many methodological insights. Her conclusions have been disputed by R.A. Brown, 'The status of the Norman knight', in War and government in the middle ages, edited by J.C. Holt and John Gillingham (1984), pages 18-32. Both contributions are reviewed in detail by Donald F. Fleming, 'Landholding by milites in Domesday Book: a revision', Anglo-Norman Studies, vol. 13 (1991), pages 83-98.
Stephen Morillo, Warfare under the Anglo-Norman kings (1994), has relevant observations on the military aspects of the role of knights. Chris P. Lewis, 'The Domesday jurors', Haskins Society Journal, vol. 4 (1995), has thrown a methodological spanner in the works with his evidence that many minor landholders are not recorded in Domesday Book at all.
Finally, John Gillingham, 'Thegns and knights in eleventh-century England: who was then the gentleman?', Transactions of the Royal Historical Society, vol. 5 (1995), pages 129-53, makes a strong case for the existence 'gentlemen' and a gentry class in Anglo-Saxon England; and John J.N. Palmer, 'The wealth of the secular aristocracy in 1086', Anglo-Norman Studies, vol. 22 (2000), pages 279-91, discusses the economic basis of the gentry classes before and after the Conquest.