21 Days of Fear by Hervé Commère 

21 Days of Fear: One Day, One Letter by Hervé Commère — a chilling YA thriller told through 21 sealed letters, with the suspense building day by day.

What would you do to bring your sister back from the dead?

Ten years ago, Lucas’s sister vanished right under his nose. The family fell apart. His mother sank into depression. His father would not speak of it. Lucas has lived with the guilt ever since — until a note arrives that changes everything.

He has twenty-one days to find out the truth. Twenty-one days to save his sister… if she is still alive. Twenty-one days to decide how much he is willing to sacrifice to bring her home.

Every action is watched. Every choice counts.

This novel’s unique format matches the countdown: each of the twenty-one chapters is sealed behind its own perforated enclosure. Open one letter a day and live the mystery in real time — or tear through all twenty-one if you cannot wait.

A gripping missing-person story packed with dark family secrets, clues, and shocking turns. Originally published in France and now in English, 21 Days of Fear is an immersive high-concept thriller for readers who want suspense they can physically unfold.