Booko.com.au

This site gives a comparison of prices and shipping costs across dozens of bookshops. I have used this site for a while and was a bit surprised it wasn't already reviewed. I use it as a first step before buying books. It is simple but highly functional. It allows a search for books, followed by verification of the edition you are looking for complete with cover design, and then it gives you a table of locations and prices. The prices cover dozens of shops across multiple countries and appear to be updated in real time. The majority of these also have links to the various shop's own websites. It is an extremely useful tool for finding the cheapest location for any given book. As an English teacher, I use it quite often. It often even compares sources of second hand copies as well. I strongly recommend trying it