I'm trying to find the title of a YA book that is about a teen girl whose boyfriend is taken by an alien. I think their spaceship is cloaked in a field near their school. The girl sneaks into the spaceship and finds out that aliens have been experimenting on people and the tests kill people. I don't think the book had a happy ending because the aliens still experiment on people and there was only one alien that helped out the girl, only because it wanted to learn more about Earth. I remember the alien calling Earth "blue planet" or something like that.
It's not a very thick book. The cover of this book is light greenish and I think it has a field on it.
I think there are more books in this series, or their are more books similar to this one since it's a very quick read and the pages were white and not cream, I believe. I'm not sure when it was published, but I read this book in the early 2000s.