There are only a handful of people allowed to totally screw with the circuits of time and space. If anyone else were to stumble upon the technology to do so the effects could be DEVASTATING.
I don't give books star ratings; I give them bite ratings. This is a bite. A book can get up to five of these evil-looking chompers, including half bites.
You can attribute your own wording to the ratings. Five's the best, three's decent, one and under isn't worth the gas to go to the library. I'll leave it to you to fill in the blanks.