Raven's Gate by Anthony Horowitz
The Power of Five #1
Slight bending on the cover - otherwise like new
 
Matt has always known he has unusual powers. Raised in foster care, he is sent to Yorkshire on a rehabilitation programme, only to find himself in the midst of sinister goings-on. Matt investigates and uncovers a terrible secret – eight guardians are protecting the world from the Old Ones, beings banished long ago by five children. But devil worshippers want to let the Old Ones back in...