Isabelle in the Afternoon by Douglas Kennedy

Paris in the early Seventies. Sam, an American student, meets a woman in a bookshop. Isabelle is enigmatic, beautiful, older and, unlike Sam, experienced in love's many contradictions. Sam is instantly smitten - but wary of the wedding ring on her finger.
What begins as a regular arrangement in Isabelle’s tiny Parisian apartment transforms into a true affair of the heart, and one which lasts for decades to come.
Isabelle in the Afternoon is a novel that questions what we seek, what we find, what we settle for - and shows how love, when not lived day in, day out, can become the passion of a lifetime.