The black room - Gillian Cross

At the end of The Dark Ground, Robert finally managed to escape from the tiny underground world in which he had found himself trapped. He needed Lorn's help to do it, and he had to leave her behind. Now, back in his own world and his own size again, he realises that Lorn will die if she has to spend the winter underground. To rescue her, he has to do two things: entrust his friend Tom with the incredible story of what happened to him, and find the 'real' Lorn above ground. Using all of their ingenuity, Tom and Rob finally track down the girl for whom they are searching. Her name is Hope, and she's been locked in a pit by her misguided parents. She is filthy, backwards, and completely dependent on others. How can she help Lorn?
Getting Hope out of her prison will take more courage and patience than the boys feel they have. And then they still have to rescue Lorn. Time is ticking away. With each passing second, Lorn feels inexplicably called by a strange force drawing her into the dangerous tunnels of her underground world while, above ground, Tom and Rob begin the desperate battle to save her.