Getting reviews for your book on Amazon is essential for any author. Like it or not - Amazon is likely to be the biggest outlet for your book sales.
Reviews drive sales - they help potential customers feel comfortable. BUT - you are now no longer legally allowed to offer someone a free, or discounted, book in return for a review on Amazon. This is a change from their previous Terms and Conditions.
Read this interesting blog from iLovetoReview. Although this company does not deal with book reviews, the blog, and the advice contained within it, is still relevant to authors.