My life was incomplete before I got this amazing book from Amazon. In terms of the medicine you'll need for the war against poverty, it has everything short of an actual modern surgery, which you cannot do much about anyway.
Superb overview of basic medicine, common diseases and working with the resources you have on site.
There are different editions depending on where you are. It's a classic work and for the price, you can't beat it. It's well-laid out and has a useful colour-code system.
A fine and admirable series of volumes