*Caesar's Commentaries on the Gallic War
(Books IV and V, chiefly 55, 54 B.C.)
*Tacitus' Agricola and Annals (chiefly from 78-84).
*Gildas' History of Britain (whole period).
*Bede's Ecclesiastical History of Britain (whole period).
Wright's The Celt, the Roman, and the Saxon.
Elton's Origins of English History.
Pearson's England during the Early and Middle Ages.
Scarth's Roman Britain.[1]
[1] The best short history.