iBookstore in my country Philippines has no new books for sale, unlike Amazon's wide variety of selection

Apple can only sell content where they have been granted licenses to sell it. You can try requesting that the books that you want to buy be added to the Philippine store, but ultimately it's the publisher's choice : http://www.apple.com/feedback/itunes.html

You can also only use your own country's store, you need a valid billing address in a country and be in that country to buy from it - the US store terms say (and other country's stores will be similar) :

The iTunes Service is available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the iTunes Service from outside these locations. Apple may use technologies to verify your compliance.

Now I'm thinking again whether I should get myself a newer iPad or just buy a Kindle Fire HD tablet, since Amazon has a larger bookstore and they have the latest titles, like Dan Brown's recently released Inferno

You can read Kindle books on an iPad by using the Kindle app for iPad.