At the Edge of the West

In the second novel of Mendoza's adventures, the 16th century girl is saved from the Spanish Inquisition only to be transformed into a 24th century cyborg (a partly artificial person). The time is now the 1860s, and Mendoza, who has never recovered from losing her lover to martyrdom in the 1540s, is in California being a botanist and living with other immortals. Suddenly a man who is her lover's exact double appears - an English spy intent on stealing California from the Americans while they are fighting their civil war.