Marlborough: ahead of his time; The Great Marlborough and His Duchess, by Virginia Cowles. New York: Macmillan Publishing Company. 456 pp. $19.95.

In old age, the first Duke of Marlborough saw a portrait of himself in full armor. ''That,'' said the admiring Duke, ''was once a man.'' According to author Virginia Cowles, the Duke was a general ahead of his time: a humane commander personally interested in his men's welfare, and the originator of the best secret intelligence network in Europe. However, she says master strategist Marlborough was more skilled in waging war than negotiating peace.

With thoughtful scholarship, Cowles paints a strong word-picture of the career of Marlborough, captain-general of England, and of political intrigue in the reign of William and Mary and Queen Anne.

The author weaves the story of the contentious Duchess of Marlborough around the Duke's, but it is his life that predominates. That's understandable. With a pair of larger-than-life characters vying for center stage, it would take two books to give equal treatment to the colorful Duke and Duchess of Marlborough.