Everything Is Fine (And Other Lies I Tell Myself) — Cathy Brett

Interwoven with tales of World War I, this is a story about growing up, moving on and the strength of a family.
Things haven't been going so well for fifteen-year-old Esther Armstrong. With her brother Max - her closest ally - absent, she's forced to face everything alone, not least her parents' heated arguments. As the summer holidays stretch endlessly ahead, she's desperate for something, anything, to divert her attention.

Then she finds some letters hidden in the walls of her family home, sent by a soldier to his sweetheart from the trenches of WWI. Esther is consumed by the mystery of these lovers - not very much older than herself - and what became of them. Perhaps in piecing together the jigsaw of someone else's life, Esther can work out how to reassemble her own, and how to make everything fine again...