Letters to the Lost — Iona Grey

1943, in the ruins of Blitzed London…
Stella Thorne and Dan Rosinski meet by chance and fall in love by accident. Theirs is a reluctant, unstoppable affair in which all the odds are stacked against them: she is newly married, and he is an American bomber pilot whose chance of survival is just one in five.

… He promised to love her forever
Seventy years later Dan makes one final attempt to find the girl he has never forgotten, and sends a letter to the house where they shared a brief yet perfect happiness. But Stella has gone, and the letter is opened by Jess, a young girl hiding from problems of her own. And as Jess reads Dan's words, she is captivated by the story of a love affair that burned so bright and dimmed too soon. Can she help Dan find Stella before it is too late?

Now forever is finally running out.