One and Only: The Freedom of Having an Only Child, and the Joy of Being One by Lauren Sandler
Sandler is an only child, and the mother of an only child. This is a part investigative work and part memoir, and she weaves in her story with facts and studies that claim that only children do just as well (if not better) than those children with siblings.
An interesting section was about how parents usually have more children so that the first kid has siblings rather than because the parents themselves actually want more children.
This was an okay book. It took me awhile to read it, and sometimes the organization of the book didn't quite make sense. If you're thinking about how many kids you want, or considering having just one child, or if you're an only child yourself, you might want to check it out.