The Watcher in the Garden

I first read this book several years ago when it was on the local school reading list. I found it was a beautiful book that stirred the imagination. It's about a fifteen year old girl who becomes enchanted with an old man's garden and who soon finds herself linked to a dangerous teen who wants the man's land for his own family, to build a garage if I remember rightly. As Kitty and the teen continue to run into each other, they begin to find out that maybe they're just two sides of the same coin.