Tightrope by Gillian Cross
When Ashley climbs up and graffiti tags a wall in the middle of the night, she has no idea how serious the consequences will be. Someone sees her, someone dangerous. Now Ashley is being watched . . . and followed. Then the notes start arriving, becoming progressively more sinister. When the threats turn deadly, Ashley realizes she can't confront the stalker alone and seeks the help of local gang leader Eddie Beale. But Eddie's help doesn't come for free-he expects something in return. Ashley has to decide how far she's willing to go-and what she's prepared to risk-to unmask her stalker.