Joan Aiken 
Shadow Guests [EPUB ebook] 

After the mysterious disappearance of his mother and older brother, Cosmo is sent away to live with his father’s eccentric cousin, and to a strange school where he is lost and lonely among his unfriendly classmates. Luckily he can escape at weekends to the peace of his cousin’s ancient mill house, and the shadowy companions only he can see. But then he learns about the family curse, with which his visitors from the past seem to be connected. The 'shadow guests’ welcome Cosmo’s help but are their increasingly menacing activities linked to his own problems?