Shelf-Worthy - These are the books that I was so into, that are just so absolutely ahmazing that you'll want to hold them in your hands and cherish every time you read them because they are just that marvelous.
Satisfying eBook - This is the rating for those books that I really liked, but I don't necessarily need to hold them. Knowing that I own them and can read them at any time is enough.
Library Grab - Books that I liked, but don't necessarily have to own at any point in my life. As long as I can borrow them -- library, friend or otherwise -- I'm good.
I rarely ever read anything that's just absolutely terrible. If I do, I will just come right out and say it, so I won't need a category for those.