Judas Tree by Simon P. Clark
Amelia Thomas, recovering from a serious head injury which has left her prone to hallucinations, leaves her native northern town for the Greek island of Voros. But the island is a sinister place full of ghosts that, although unseen, make their presence felt through the minds of the inhabitants. Gradually, the sanity of those who live there is eroded.. Visitors who stay for any length of time rarely leave and Amelia comes to realise that she must either escape this haunted place or stay there forever.