I give each book a grade of A-F, often adding a + or -.
A=Extraordinary Book. Plot is original, language is beautiful or subject really grabs me. Very few books get As.
B=Good Book. Most books reviewed here will get a B of one ilk or another. That means I enjoyed the book, as I enjoy most books I read. Generally a B- means there is either something I didn't like about the book or that the plot is just too formula. Generally a B+ means I really enjoyed it, but there was nothing about the book to move it into an extraordinary category.
C=A book I didn't care for, usually for a stated reason, but which you might like. Few books will get this grade simply because I've quit finishing review books I don't like.
D, F If someone insists of a review of a bad book I suppose I could still give these grades, but I'm a lot more likely to give a DNF and explain why.
I wrote this post about negative reviews. Generally speaking, books I am not enjoying reading are DNF'd after 50 pages. Generally speaking, I don't review them; however, sometimes I do and that post should let you know when a negative review is likely.