Here's how it works: Offer your own books free of charge, decide whether you're up for shipping domestically or worldwide, and in return, you can have books sent to you, free of charge. I LOVE IT!! Sharing books = happy, literate people!! It's a points-based system, so you can't just mooch all the books you want without donating some of your own. There aren't a lot of users within Mexico offering books in English, but if you register, there will be more. :)
I registered and posted one book today, and already, somebody is trying to mooch it off of me. Brilliant--mooch away!!
**By the way, if you live in Mexico, have kids, and want some English language children's books, I am considering parting with some of my stash, especially my chapter books. Shoot me a comment, let's exchange datos, and the book fair-y will be on its way.