We can't give you specifics, because we have no idea how your system works.
For example, you could be applying a discount value as a percentage in each country, so Mexico would need a discount value associated with it of 13.5%
But if each country needs a separate price for each book, then you need a much bigger system with lookup codes for each book, for each country - and with 242 countries, that's getting complex to administer in terms of data entry alone!
But basically the process is the same:
1) Identify which country the user is in. You can do this from the IP address, but it's not very accurate:
Using IP based Geolocation - and why it's pretty much useless.[
^] - it's better to identify the country from the user shipping address, ratehr thna rely on electronic identification.
2) Get the book price for the country and book. This will depend on how you work out teh price!
3) Don't forget to allow for different shipping costs / methods - some countries are harder to ship to and that means more cost. Remember local import duties may be applied.