An awful book that has offended me on some level. Do not engage.
A bad book. I’m warning you- read only if you absolutely, positively must.
A book that’s good, but not great. Read it if it interests you.
A very good book, although it’s missing that little something that could push it into greatness. Definitely worth a read.
A book with which I am in love. You have to read this book.
The Literary Omnivore 
I get the need, but it still irks that one STAR (whoops, shouting, sorry) is somehow distasteful. Like if one smiley-face actually meant I’m frowning. But of course, negative stars are not supported. Sadly.
One star is giving the author credit for writing a book and getting it published. It’s like getting five points for putting your name on the test; it’s the barest minimum. Zero stars would be reserved for the kind of book someone talks about but never commits to paper, because the effort is not made. Hope that helps.