Jonah Berger, Assistant Professor of Marketing, The Wharton School, University of Pennsylvania, Alan T. Sorensen, Associate Professor of Strategic Management, Stanford Graduate School of Business and Scott J. Rasmussen, an alumnus of Stanford University conducted a study evaluating the effects of positive and negative reviews/publicity on overall book sales. The surprising results? 

"Regardless of whether the book was written by a new or established author, being positively reviewed significantly increased sales; a positive review generated between a 32% and 52% percent increase in demand (no significant difference between coefficients, z = 1.0, p > .30) In contrast, estimates indicate that the effect of negative publicity depended on existing author awareness (z = 2.30, p < .05). For books by established authors, a negative review led to a 15% decrease in sales (this estimate is slightly imprecise due to the relatively small sample size). For books by relatively unknown (new) authors, however, negative publicity has the opposite effect, increasing sales by 45%." 

Read the complete study.