Posting a review on Amazon.com is one of the nicest things you can do for an author when you read a book and like it, or find value in its content. Amazon.com dominates bookselling; its reviews influence purchasing decisions made far beyond the Amazon.com stronghold, extending into brick-and-mortar stores and to competing online sellers. Positive reviews increase an author's sales and have direct bearing on an author's willingness and even ability to create more of the content that you like and benefit from.
Figure 1 shows an example to highlight my point. The blue line shows the shape of sales for books on Oracle Database topics published by Apress in 2013. Overlaid on the chart are pink and green lines corresponding to the number of reviews and their average star-values. There are nine books represented that are listed from left-to-right in random order. You can readily see the imperfect but still noticeable correlation between reviews and sales.
Look at the peaks in the sales line represented by Books 2, 5, and 8. Those are the best-selling of the lot, and each is accompanied by similar peaks in review count and star-value.
Also notice the review-count valleys under Books 4 and 7. Those two books have zero reviews, and their sales are correspondingly low.
Book 1 is an interesting anomaly with its low review count and strong sales. The count is non-zero however, and the star-value is high.
Book 6 is also an interesting anomaly with a very good average review and yet low sales. No correlation is perfect.
You've already paid for your book of course, and the author has benefited from that payment. Truly you're under no obligation to review, despite my title on this post. Yet most authors will affirm that gaining a reasonable return on their time spent writing is a difficult, difficult thing. (This is true in fiction as in non-fiction). Authors write for the inner satisfaction of being creative and helping their colleagues. Support those authors! Support the ones creating the content that is helping your career. Take five minutes. Visit Amazon.com. Write a review. Make it a good one.