The Halfmen Of O

When Nick Quinn goes to spend a couple of weeks on his uncle's farm at Lodestone Creek, he expects it to be like every other summer holiday. His dopey cousin Susan is more remote than ever, but swimming and exploring will make up for the lack of companionship.
But Susan is spirited off down a disused mineshaft into the world of O, and is taken prisoner by Odo Cling and the Deathguard. Nick follows to rescue her and the Woodlanders of O come to his aid. They know that Susan is the only one who can save their world, and perhaps her own.