The Marriage of Sticks by Jonathan Carroll
Returning to her class reunion, Miranda Romanac has her heart set on meeting James Stillman, her first boyfriend, once again. Her life's never quite measured up to the ideal he represented for her but she is devastated to learn that he died three years before, in a car crash. Her life settles back into routine in New York, and she meets the fabulous Frances Hatch, mistress of many of the great in Paris in the twenties, and at the same time starts an affair with a married man. At which point she sees James Stillman, waving to her across the street. And her life changes forever. Confronted, literally, by the lives she's ruined one way or another, she learns the horrifying truth about herself and her own immortal existence.