The author is a British economist and has done extensive research on migration. In the book, he basically shows under which conditions migration can be beneficial for a Western nation and under which it will rather accelerate the decline of the host nation.
Needless to say, his findings are pretty obvious but nevertheless, it is a very good read. He summarizes some interesting studies. Among them, the Putnam study from the NYT article above.
This guy, Putnam, is a hardcore Harvard Liberal. He conducted the largest social study ever done in the US. His point was to prove that diversity was indeed our strength. Well, his finding showed a completely different picture. He was so horrified by the results that he didn't believe them. Thus, he included more markers into his study, hoping that by digging deeper, the picture would change. It didn't. In fact, it became clearer and clearer that diversity = disaster on many social levels. Since this went against his bias, he considered not publishing his study. Eventually he did, though.
Another study cited looked at the diplomatic personnel of the UN in NYC. They gathered data on which nation's drivers would pay the parking meters. Since they have diplomatic immunity, none of the drivers have to pay them. The outcome, surprise surprise, was that drivers from countries like Nigeria, Saudi Arabia, etc. hardly paid them, while drivers from countries like Norway, Germany or Japan would pay up.
The author makes the point that you cannot import a bunch of people from such nations and expect them to comply to our Western norms. By not regulating their influx, he argues, they'll destroy the host nation's social fabric if they reach a certain threshold in the population. Basically, what we are witnessing right now in many Western nations.