There's nothing more subjective than a book rating system.
After all, it's really based on whether or not you like that type of book, plus how well-written it is, plus characterization and movement of plot and all that good stuff.
But I've noticed that I have my own way of classifying what I read. And so, if nothing else, it gives you a good idea of how I felt about the book.
So without any further ado:
A book that looks good, has a great description, but I couldn't get past about the 1st chapter -- usually due to sex or language. I don't have any examples for you at the moment because I never finish those books and so don't remember them!
A book that takes work to get through either because the plot is confusing or the subject matter has a lot of stuff in it that I am not familiar with. The Life of Pi was a workable book for me. I finished it, but felt it was more of a chore than a pleasure.
A good book sit's on my nightstand usually. I read a couple chapters before I go to bed each night. The story is easy to keep in my head if I pick something else up in the meantime, but I'm content to read it when I have the time. The new Kane Chronicles from Rick Riordan have been like that.
These are the books that I can't put down. They're the owns literally that are attached to me -- I read them while I eat (a very very bad habit), on the couch while the girls watch a movie, in the bathroom (sorry, you maybe didn't want to know that). I shoot through them until they are done! That's what happened with The Hunger Games trilogy by Suzanne Collins. Although I will warn you they are incredibly violent.
These are the books that, even if I picked it up at the library, I will then buy so that I can read them over and over whenever I want to. I also hesitate to lend these books to friends because I fear that I won't see them again! These is My Words by Nancy Turner is in that category. In fact, I finally went and bought ANOTHER one because my mom didn't give mine back (or did she send it back to Grandma because I wouldn't give it back?). Anyway, for children's books, these are the ones that my girls ask me to read over and over and over -- thus they are personal library worthy
So that's it in a nutshell. Of course that doesn't address content AT ALL -- this spans all genres really. But it gives you a better idea of how I feel about a book. And so if you find your taste runs like mine, it could be really helpful!