I read a lot of books. I also rate and review a lot of books. Each book site has their own system for rating. For example, at Amazon.com, when you rate a book with 5 stars, it means you "loved it." At goodreads.com, a 5 star means "it was amazing." Unfortunately, this can lead to some variability from site to site. Furthermore, sometimes the options are too restricted for the reviewer. Finally, if you have read reviews of books, it is often like the grading cycle in the modern American university. In other words, there is "grade inflation." Each of these variables contributes to diminished meaning when it comes to book ratings.
My own rating would look something like this:
1=This book is actively destructive. As one example, it may present a viewpoint well that is actively harmful to unsuspecting readers. Like a ouija board, it could do real damage. A second example is something that is so poorly written or constructed that the reader loses brain cells in the process. Much like Billy Madison's principal, "Mr. Madison, what you've just said is one of the most insanely idiotic
things I have ever heard. At no point in your rambling, incoherent
response were you even close to anything that could be considered a
rational thought. Everyone in this room is now dumber for having
listened to it. I award you no points, and may God have mercy on your
2=This book was fair. In a world where there are so many good things to read, why would you bother with this one? Put it away and pick up something better. It would even be better to re-read something you have already read than to read this one. Even vaccuuming would be preferable.
3=This was a good book. I enjoyed what I read. I learned a little. However, there were some things about it that kept its rating down. Its applicability is likely to be short lived or it may speak to just a limited audience despite higher intentions.
4=This book was really good. It won't make my must read list, but I think most people would be better off by reading it. It may not become a time-tested classic, but it is important enough to highly recommend.
5=This book is a classic and will continue to be for the foreseeable future. This book should be read repeatedly and be anchor for your bookshelf.
Most of the books I read would be in the 2-4 category. It is a rare book that would qualify for a 1or a 5.