Santa Montefiore - The Summer House

Antoinette's world has fallen apart: her husband, the man she has loved for as long as she can remember, has died tragically in an accident. He was her rock, the man she turned to for love and support, the man she knew better than she knew herself. Or at least so she thought...
For as she arrives at the familiar old stone church for George's funeral, she sees a woman she has never met before.
Phaedra loved George too, and she could not bear to stay away from his funeral. But as she sits before his wife, she knows that what she is about to reveal will change all their lives forever.

This book is published in the US under the title The Girl From Paris