By David McKissack — 7thVirginia, Continental Line Saratoga, by Richard M. Ketchum. Hardback. Henry Holt and Company, New York. 1997, 545 pp. $30.00. ISBN 0-8050-4681-X. On June 20, 1777, a body of 7,000 of the 18th century’s most disciplined and experienced troops began a campaign in northern New York which was designed to end the…