First-Person, Tom Ward's perspective
The Spook's apprentice, Tom Ward, narrates the book for us. And, as we find out at the end, he's actually been writing it all down for us. So Revenge of the Witch is kind of like Tom's diary.
In any other book, we might be concerned about an unreliable narrator. But not this one. Tom does say, "I've written most of this from memory" (Epilogue.1). However, we trust him to tell the truth because Tom makes no effort to make himself look perfect.
When he's scared, he tells us. When he's lonely, he tells us. When he frees a witch because of his own stupid mistake, he owns up to it. And even if he is fibbing a bit, well, we don't care.
It's a lot more interesting to read his book than it would be to read our own journals, which just have We Heart Donuts scribbled in all the margins. Oh, and We Heart Oliver Wood, obviously.