Many publishers will specify (in the approval notification email) when to publish your review – for instance, “please hold all reviews until one month before the publication date.” Or they may note a preference in their specific Approval Preferences (on their Browse Publishers page).
If the publisher hasn’t specified, then you’re free to publish your review whenever you’d like, although readers might find it more useful if you wait until the book is closer to publication. Keep in mind that most publicists want reviews and other coverage of books to happen just before or once the title is on-sale, so that anyone hearing about the title will be able to pre-order or purchase it right away.
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