Stephen Burt, as part of a series at Lemon Hound, offers some sound ideas on the art and craft of reviewing.
Reviewers should describe the book accurately in a way that makes clear which (if any) readers will likely enjoy it; reviewers should say what’s interesting, what’s well done, what stands out (for good or ill) in a book. If the book, or its author, or books much like it, have already attracted attention, reviewers might also say why; if the book (e.g. almost all first books of poetry) hasn’t attracted much notice as yet, reviewers should explain why it deserves notice, why it’s worth our time (if indeed it is).