Marion Zimmer Bradley's Mists of Avalon retells Arthurian legend from Morgana's point of view. Kenneth C. Flint's series (generally found in the science fiction section) has two books of interest: Challenge of the Clans about Finn MacCumhal, and Storm Upon Ulster, a retelling of the Irish epic Tain bo Cualnge and the relationship between Cuculain and Maeve. The life of St. Patrick before he went to Ireland is the subject of The Last Rainbow by Parke Godwin. Anya Seton, who has written many historical novels, tells of Bodiccea and the Romans in The Mistletoe and the Sword. Hebe Weenolsen's The Forbidden Mountain is a fictional account of the building of Stonehenge.
John Williams' Augustus is a fictional biography of that emperor, and Taylor Caldwell's A Pillar of Iron, is a very detailed novel about Cicero.