Climbing a notch to the No. 1 nonfiction spot this week is In Retrospect: The Tragedy and Lessons of Vietnam, former defense secretary Robert McNamara's mea culpa about the Vietnam War. Writes McNamara, who has kept publicly mum about the conflict for nearly 30 years: ''We of the Kennedy and Johnson administrations who participated in the decisions on Vietnam acted according to what we thought were the principles and traditions of this nation ... Yet we were wrong, terribly wrong. We owe it to future generations to explain why.'' The book has already been to press seven times, for a total of 245,000 copies in print.