Rural essays explore inner self; Letters From the Country, by Carol Bly. New York: Penguin Books. 184 pp. $4. 95.

This unusual collection is of stern, perceptive, and loving short essays that originally appeared in the Minnesota Monthly. Bly is at her best, her most scrupulously perceptive, when exploring the inner lives of those who are isolated in the country. Her essay ''Great Snows'' will bring knowing smiles to all who love blizzards. ''Quietly Thinking Things Over at Christmas'' is a small masterpiece. Some of the pieces emphasize the spiritual side of rural life, others the social side, but it is Bly's great strength to see both sides as inseparable in the everyday world.