Say a book has 10 reviews on amazon.com and 2 reviews each on amazon.co.uk, amazon.it and amazon.in, is there a way to aggregate all 16 reviews?
Amazon Product Advertising API License Agreement prohibits use of the Reviews data for any purpose other than driving sales on the Amazon Site. So I doubt if any site can lawfully provide the aggregated info. You may have to scrape & compile it yourself for your personal use by utilizing Amazon's consistent URL pattern for a product review page of each country.
Typically, the URL of a country-specific Amazon Reviews page for a product has this pattern:
For example, the reviews for the book with ASIN or product code of 0143031031 on Amazon.in are available at this URL: http://www.amazon.in/product-reviews/0143031031/