Name: The Tulip Touch
ISBN: 9780141320472

Anne Fine

Tells of a strange and disturbing friendship, seen through the eyes of Natalie, as she gets to know Tulip Pierce, a delinquent girl most others go out of their way to avoid. This work explores the dark side of a friendship bordering on obsession, and depicts one girl's gradual decline into hostility and violence. Ages 10-13.

The Tulip Touch

Award winner-1996 Whitbread Children's Book of the Year.

Caitlyn Johanna Bolton 

The first time I read this I was 10 and it has stuck with me so strongly in the past 10 years that I had to pick it up again. Similarly to The Ocean at the End of the Lane by Neil Gaiman this is both a children's story and an adult story, with so many themes that only adults would be able to fully comprehend. 
It is definitely one of the darkest books I read in my childhood, but I will never regret it as I don't think I would have picked it up now, at a time when I can understand and appreciate it for what it's worth, if it wasn't a reread.