I'm now on a quest to read books about only children. I'm probably biased on this one because one of the authors is a close colleague.
I really liked this one, but again, it could be because I am comfortable with the writing style because of my connection to one of the authors. But, more so than One and Only, I felt like this book spoke to my concerns--even explicitly noting negatives along with the positives.
The downside: this book is quite dated, though I am aware of more recent research that only supports the "good" findings in this book.
This is definitely written for an audience of parents of only children (or those in the process of deciding if they will have more than one child)--and I think it's an excellent read, despite the fact that it is so dated.