Dream involving school run by people possessed by demons?

The only reason you had that dream is because you read that book. If you didn't there would've been a bigger chance of you not having this dream. A brain is a memory file. It stores every single memory of you reading a book or watching television even when your not paying attention your brain will still do that. In your dream your brain picks up memories from your brain and makes it into a dream. And that's the reason why you had yours it's cuz of that book you read.