This is probably well known by most, but Oxfam have an online bookstore, Good Books, where "all profits go to Oxfam to help fight its global battle against poverty and social injustice."
There is NO extra cost to the customer as they don't mark up their books to cover this contribution. According to them, "our prices remain among the lowest you will find; delivery worldwide is completely free, and with over two million titles in stock our range is one of the largest you will find."

And when I read updates like this one about a child trying to express the horrors they've seen/experienced in Darfur, it really makes the heart go out to them as well as wanting to help those e.g. Oxfam who can help them.

So, if you're about to buy a book, how about checking to see if it's available at Good Books first? : )
Every cent, every dollar counts and has a far bigger trickle down effect.