Sophie Hardach’s thought-provoking third novel is one of four books shortlisted for the 2019 Costa novel award, to be announced next week . Born a decade before reunification, Hardach grew up near Frankfurt in West Germany; her book focuses on a family living on the other side of the border, in the GDR.

Now settled in London, adult siblings Ella and Tobi Valentin seldom talk of their childhood in East Berlin. While both have vivid and disturbing memories of their family’s failed attempt to flee across the Hungarian border into Austria in 1987, they know very little of their mother’s subsequent imprisonment and nothing at all about the fate of their baby brother Heiko, who was taken away by the authorities and never seen again. When their mother dies, leaving behind some tantalising scraps of information, Ella returns to Berlin, hoping to find answers.

In the Stasi archive, newly open to the public, she meets Aaron, an intern whose main task is the reconstruction of thousands of files shredded in the final days of communism, strip by painstaking strip. Despite his best efforts, Aaron can never be sure he has put the documents back together correctly; vital pieces are often missing.

Hardach builds up her narrative in the same way, starting from the edges and working in, the mystery revealing itself in fragments as she moves between Ella’s childhood memories and the complicated truths she begins to uncover. Through Ella’s investigation, one of many thousands being pursued through the archive, the novel peels back the contradictions that endure in a Berlin still coming to terms with reunification. It is a city where brutal prisons have reinvented themselves as museums and their interrogators as ordinary citizens, where the archivists filing documents about Stasi atrocities are themselves ex-Stasi, and decent people are determined to forget the things they did to survive. Confession With Blue Horses might have been another cold war thriller. Instead it is an absorbing slow burn of a book that not only casts light on Germany’s recent history but depicts, with careful tenderness, a family simultaneously torn apart by ideology and bound by powerful ties of love.

• Confession With Blue Horses is published by Head of Zeus (£8.99). To order a copy go to guardianbookshop.com. Free UK p&p over £15.