Douglas Richardson, Royal Ancestry: A Study in Colonial and Medieval Families, 2nd edition (Salt Lake City, Utah, 2013), volume IV, page 561

4. BEATRICE DE MANDEVILLE, born about 1105 (aged 80 in 1185). She married (1st) HUGH TALEBOT, from whom she was divorced. She married (2nd) WILLIAM DE SAY. They had two sons, William and Geoffrey. WILLIAM DE SAY was killed at the Siege of Burwell Castle, Cambridgeshire in August 1144. His widow, Beatrice, was heiress in 1189 to her nephew, William de Mandeville, 3rd Earl of Essex. On account of her great age, her only surviving son, Geoffrey de Say, was allowed to take her place, and to have seisin of the inheritance. His fine for succession was fixed at 7,000 marks, but he did not part any part of it, and was accordingly disseised. Geoffrey Fitz Peter, husband of Beatrice, elder daughter of her eldest son, William de Say, then gave 3,000 marks; in 1190 he was granted seisin of all the lands of Earl William de Mandeville, his wife actually being the nearest heir. Beatrice died 19 April, in or before 1197, probably at Rickling, Essex where she resided. She was buried in Walden Abbey, Essex.

Brydges Collins' Peerage of England 7 (1812): 16-39 (sub Twisleton, Lord Say and Sele). Baker Hist. & Antiqs. of the County of Northampton 1 (1822-1830): 544-545 (Mandeville-Fitz Peter-Bohun ped.). Clutterbuck Hist. & Antiqs. of Hertford 3 (1827): 190-194 (Mandeville-Say ped.). Lee Hist., Description & Antiquities of ... Thame (1883): 332 (Mandeville ped.). VCH Hertford 3 (1912): 319-323, 332-347. Genealogist n.s. 34 (1918): 181-189. VCH Buckingham 3 (1925): 141-155; 4 (1927): 100-102. C.P. 5 (1926): 120-122 (sub Essex). Paget (1957): 485:1. VCH Middlesex 5 (1976): 224-229. Tanner, Fams., Friends, & Allies (2004): 302 (Mandeville ped.). Harper-Bill & Vincent Henry II: New Interpretations (2007): 110, footnote 3.