''Stag's Leap'' is stunningly poignant sequence of poems that tells the story of a divorce, embracing strands of love, sex, sorrow, memory, and new freedom. In this wise and intimate telling--which carries us through the seasons when her marriage was ending--Sharon Olds opens her heart to the reader, sharing the feeling of invisibility that comes when we are no longer standing in love's sight; the surprising physical bond that still exists between a couple during parting; the loss of everything from her husband's smile to the set of his hip. Olds is naked before us, curious and brave and even generous toward the man who was her mate for thirty years and who now loves another woman. As she writes in the remarkable ''Stag's Leap,'' ''When anyone escapes, my heart / leaps up. Even when it's I who am escaped from, / I am half on the side of the leaver.'' Olds's propulsive poetic line and the magic of her imagery are as lively as ever, and there is a new range to the music--sometimes headlong, sometimes contemplative and deep. Her unsparing approach to both pain and love makes this one of the finest, most powerful books of poetry Olds has yet given us.