''A comfortable chair and a Mary Stewart: total heaven. I''d rather read her than most other authors.'' Harriet EvansThe rambling house called Thornyhold is like something out of a fairy tale. Left to Gilly Ramsey by the cousin whose occasional visits brightened her childhood, the cottage, set deep in a wild wood, has come just in time to save her from a bleak future. With its reputation for magic and its resident black cat, Thornyhold offers Gilly more than just a new home. It offers her a chance to start over. The old house, with it tufts of rosy houseleek and the spreading gilt of the lichens, was beautiful. Even the prisoning hedges were beautiful, protective with their rusty thorns, their bastions of holly and juniper, and at the corners, like towers, their thick columns of yews.