I have an idea for a book but im not sure if its good or not..?

I think it is the start for a good idea. It has a nice backbone, but you need to make sure that it doesn't become overly cliche. Don't make Jess all superhero-y and don't make her ignore things that real people try to do, like contact the cops if she finds out something. To keep it so the cops don't solve the mystery and Jess does, maybe they don't believe Jess because she has a reputation of sending false alarms, or something along those lines. Good luck, and I hope the twist is great!