Richardson, Douglas. Royal Ancestry: A Study In Colonial And Medieval Families, in 5 vols. (Salt Lake City, Utah, 2013), Vol. I. page 571.
Burke, B. (1866). "Aton: Barons de Aton," in A Genealogical History of the Dormant: Abeyant, Forfeited, and Extinct Peerages of the British Empire, pp. 15. Google Books.. Burke (1866), states that Isabel Percy is the dau. of the 3rd baron... but it might be second (see Lewis, n.d., and Wikipedia: Henry de Percy, 2nd Baron Percy).
Lewis, M. (n.d.). "Anastasia de Aton #11869, d. after 22 July 1389," (citing Richardson and Cokayne). ORTNCA. Web.