I use a 5 out of a 5 rating system because I believe it is the simplest way of rating a book. Furthermore, several review sites use some format of a 5 out of 5, whether it be stars or a physical rating.
5 out of 5: Extraordinary. Loved it, mind blowing, just over all in its own class.
4 out of 5: Compelling and a page turner. An enjoyable read.
3 out of 5: Good. Nothing particularly outstanding about the book, but it wasn’t bad.
2 out of 5: It was okay and problematic. May struggle to finish.
1 out of 5: Would not recommend, poor and lost interest.