The French Agent by Belinda Alexandra

Publisher's Synopsis

Paris 1946:
Sabine Brouillette is a war crimes investigator with the French secret service. She lost her family, including her young son, when her Resistance circuit was betrayed near the end of the war. New evidence comes to light that the traitor was a British double agent who went by the codename 'the Black Fox'. Now her quest for revenge has a single focus: find the Black Fox and kill him.

Sydney 1946:
Landscape designer Diana White has been waiting six years for her husband, Casper, to return from the war in Europe. Her son, Freddy, was only a baby when his British-born father joined the RAF. But Casper is a changed man when he returns from the convalescent hospital in England where he has spent the past year under mysterious circumstances. No longer the easygoing personality Diana fell in love with, he is now darker and more secretive.

Soon Sabine and Diana find themselves on a collision course - one seeking vengeance, the other willing to go to any lengths to protect her family.