The Hangman's Garden

Nora Brooke drowned last year. Her heart stopped: long enough to leave, deep inside, a shadow of death that opened her to other shadows.
Coming to Holdfast Island for remedial school, Nora finds a garden in the woods: an old, overgrown prison yard. She enters by a gate that then vanishes, glimpses a restless figure who is suddenly gone, and finds thousands of blue flowers like watching eyes.
She finds friends, especially irrepressible Jack McKie. And Adam: seen in the prison ruin and met underwater while swimming, when Nora nearly drowns again. A local boy, they think, until they learn of an old tragedy. Three teens died here: Ursula stabbed, Graham hanged, Adam drowned. Triple suicide? Or murder? Adam says he loved Ursula but lost her. Now he's bound here.
Questions swarm. Why does Nora sleepwalk nearly to her death? Why is Jack suddenly accident-prone, like Graham? Who haunts the prison, where the warden, Adam's grandfather, was called the Hangman? What will happen if Nora sets Adam free?