1 STAR -- Disliked it pretty thoroughly but it may still be worth a read; if so, I'll state why.
2 STARS -- Disliked it but making an allowance (of one star) on account of the author's good standing or some other caveat.
3 STARS -- Liked it enough to finish it but with noted reservations.
4 STARS -- Thoroughly enjoyed it for whatever reason (literary or otherwise) despite any shortcomings.
5 STARS -- An outstanding and literary read that ranks among the best of its kind, whatever the kind may be. No shortcomings that amount to more than quibbles.
No stars means I didn't read enough of it to justify any, or, to put it another way, the book didn't interest me enough to warrant any stars.
Most of the books I post have high ratings because I seldom bother to review books I just don't like or can't finish.