Mr. Singer is far from the world’s only serious thinker about poverty, but with “The Life You Can Save” he becomes, instantly, its most readable and lapel-grabbing one. This book is part rational argument, part stinging manifesto, part handbook. It’s a volume that suggests, given that 18 million people are dying unnecessarily each year in developing countries, that there is a “moral stain on a world as rich as this one.” We are not doing enough to help our fellow mortals.