The Black Room — Gillian Cross

Robert has come back from a horrible ordeal in a strange parallel world. Now he can't stop thinking about the people he left behind to face a winter that most won't survive. When Robert sees a chance to save his friend Lorn, he is desperate to succeed. But is he prepared for the realities of her world? Meanwhile, Lorn struggles to lead the little band of survivors, even as she feels herself drawn to a secret tunnel where an unknown menace lurks.
Seamlessly blending fantasy with a hard-hitting survival tale, Gillian Cross has created a completely original psychological thriller.