The jury calls the novel "a breathtaking debut", and praises its blend of suspense and history, writes BBC News.
Every word is perfectly placed, and page after page reveals an aspect of war and the Holocaust that has so far been quite unexplored in fiction, says the jury chairman, Kit de Waal.
The story follows a reserved woman whose brother asks if his girlfriend can move in with her for the summer. Despite her initial reluctance, a closer relationship gradually develops between the two women.
The prize for non-fiction, Women's Prize for Non-Fiction, went to Rachel Clarke, whose book "The story of a heart" deals with how an organ donation is experienced by two families.
The winners are awarded each approximately 380,000 kronor.
