The Ropemaker — Peter Dickinson

Tilja has grown up in the peaceful Valley, which is protected from the fearsome Empire by an enchanted forest. But the forest's power has begun to fade and the Valley is in danger. Tilja is the youngest of four brave souls who venture into the Empire together to find the mysterious magician who can save the Valley. And much to her amazement, Tilja gradually learns that only she, an ordinary girl with no magical powers, has the ability to protect her group and their quest from the Empire's sorcerers.