This is a pretty interesting question, and the answer can be applied to any type of site that claims to compare prices. After all, all sites that successfully compare prices have to work on the same basic principle, if they do not, then they do not really work.
Every book aimed for large scale distribution is printed hundreds of thousands, or even millions of times. In each printing (or edition), a book is marked with a number, an ISBN. This is either a 10 or 13 digits number that identifies this book, and that can then be used to search for this book in databases, or to compare the price of book “A” with book “B” and you can be sure that if two books have the same ISBN than they are the same book. So the best way to make sure you are buying the good book is by verifying its ISBN.
It is at this point that we come in. In the past it was difficult to compare prices because you had to go from store to store, but the Internet has allowed for increased transparency, putting power into the hands of the smart consumer. Of course, the seller or store needs some kind of online presence and an online catalog of books for search engines to work.
Given that books are sold at different websites by different sellers at different prices the book price comparison engine searches through the largest bookstores on the web, to find you the best deal on the book you want. There are two steps involved in this: the search and the comparison.
The search is exactly what it sounds like. A request is sent to each and every bookstore for books that match the search criteria entered by the user. Let’s say you want a book on “statistical business analysis”. A request is sent to bookstores to give a list of “statistical business analysis”, or with “statistical business analysis” as the subject. Every bookstore then returns a list of books (or none if none is found). The search engine will then order these results according to an algorithm; for example a book titled “The analysis of quarterly results’ statistical data – Business” will be lower in the list than a book titled “Statistical Business Analysis applied to real-life scenarios” because the second book’s title matches exactly the search criteria. Once sorted, the results are displayed to the user.
Once the user picks a book to get the best prices for it, the second step, the comparison, begins. A request is sent to all bookstores for the current price, availability and shipping price for a given book. The results are then sorted according to total price and the result is displayed to the users.
So why are there so many different book price comparison websites? The difference is in the details. Some might be very fast, but they search and compare using internal databases instead of live data, something that might turn out to be a problem. Indeed, since the price and availability of books sometimes fluctuate wildly, using information a few days, or even a week old, might mean inaccurate results. You should always use engines that fetch live data as it is guaranteed to be the most accurate; it is true they are a bit slower but waiting those few seconds more might save you a lot of trouble.
Other search engines will display a lot of details when what you really want is the book. At We Compare Books we aim at simplicity. Just search for a book, click on it to compare prices and you are ready to buy it. Three mouse clicks is all you need to find the best price.

An example of result you get with We Compare Book
If the user would like to see all prices for bookstores that currently have the book, simply clicking on the “Check full comparison report” link will bring up a list of all bookstores and their price for the book. Sometimes you might want to buy from a particular bookstore for a variety of reasons; by looking at the list you can quickly decide if it’s more important to buy from that bookstore or to buy for the lowest priced one.