Collins, Arthur, and Egerton Brydges. "Percy, Duke of Northumberland." Collins's Peerage of England Genealogical, Biographical, and Historical. Vol. II. London: Printed for F.C. and J. Rivington, Otridge and Son ... [et Al., 1812. 217-366. Print.
It may be possible to confirm family relationships with Richard by comparing test results with other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree: