Marcus and the Shadow by IOAKIM IOAKIM

Marcus notices the shadow on a Tuesday in October. It does not stop when he stops. It reaches forward toward something he cannot see. It points at walls. It knocks over glasses. And it will not leave him alone until he understands what it wants.
Behind three layers of wallpaper in his bedroom wall, Marcus finds the answer — the hidden notes of Thomas Vale, a boy who lived in the same room in 1914 and disappeared before finishing something that mattered. Thomas had discovered a serious injustice done to a local family. He had been thirteen days from completing his evidence when he ran out of time.
Marcus picks up where Thomas left off. He searches archives, finds hidden documents behind a false wall in a century-old cellar, and eventually brings the truth to an eighty-one-year-old woman who has waited her whole life to hear it — because her great-great-grandmother always said it had been taken, and no one ever believed her.
When it is done, the shadow is his again. He tells no one. He needs no one to know.
A beautifully crafted story about inherited responsibility, quiet justice, and the profound dignity of doing the right thing without an audience.
Perfect for independent readers aged 10-13.