One weirdly striking thing missing from Caplan's book and this review is one of the most common objections people have to mass immigration: loss of their dominant culture.
Given how much anti-immigration rhetoric focuses on precisely this argument, it is bizarre for Caplan not to take it seriously and makes me concerned he is living in an academic bubble so heavily biased towards pro-immigration arguments that he's failed to even acknowledge it as a concern.
Consider that many people in the UK/Europe bemoan that their major cities have entire sections with no native speakers, are full of Arabic/Polish/Chinese signage or whatever, and bear no resemblance to the place they grew up in.
Lots of anti-immigrant US groups online also fear the the displacement of Christianity (or Judeo-Christian culture) as the major value system in America. A less controversial version of this argument may just be that people value shared continuity/history with their fellow countrymen and enjoy having a sense of kinship with people who share their cultural background.
I'm not saying this the best argument, nor am I agreeing with it, but it is extremely common.
We can hand-wave this away, or say it isn't important, but it is a problem that people viscerally feel and if you're going to argue for something as radical as open borders you better have a good argument for why those people should ignore the feeling that their own cities don't feel like their own country the more mass-migration occurs.It may be simply that the pro-immigration arguments outweigh this concern, but again, Caplan needs to convince Middle America of that and at least take it seriously.
Until you do, the economic case will never persuade anyone.