The Ghost Prison by Joseph Delaney

Eerie and atmospheric with a monster at its heart, this little gem of a ghost story is guaranteed to haunt long after you've put the book down From the bestselling author of the "Spooks "series.
"'This is the entrance to the Witch Well and behind that door you'd face your worst nightmare. Don't ever go through there.'"
Night falls, the portcullis rises in the moonlight, and young Billy starts his first night as a prison guard. But this is no ordinary prison. There are haunted cells that can't be used, whispers and cries in the night... and the dreaded Witch Well. Billy is warned to stay away from the prisoner down in the Witch Well. But who could it be? What prisoner could be so frightening? Billy is about to find out.
An unforgettable ghost story from the creator of the Wardstone Chronicles ("Spook's Apprentice") series."