I have a link on my sidebar for a Free Book on missions. It's excellent. I highly recommend reading it, even if missions isn't your "thing" - the author's personal testimony is just amazing and is guaranteed to encourage your faith.
BUT, I would say that the only weakness of this book, which focuses on the Third World reaching their own for Christ, is something I could not quite articulate myself. The following video message from John Piper does an excellent job clarifying why the age of Western missions is NOT over - why it can not be...
...and another entitled "Keep the Missionaries Home in America?":