Alvaro Vargas Llosa’s timing is as superb as his book, which lands smack in the middle of a feverish Washington debate over America’s most recent arrivals. Our current immigration policies have contributed to labor shortages; created thriving markets in human smuggling and document fraud; and left us with 11 million-plus illegal aliens. Sen. Marco Rubio’s immigration-reform bill is the latest effort to overhaul the system, and we’ll find out in the next few weeks whether he can muster enough support within the GOP to succeed.

These days, immigration reform is less a partisan issue than a Republican family squabble. Free-marketers argue that U.S. immigrants are catalysts for economic growth and innovation, while restrictionists maintain that Latino foreign nationals threaten America’s value system and burden an entitlement state already on the verge of insolvency. What recommends Global Crossings is that it offers a thoughtful critique of the restrictionists from the standpoint of a fellow conservative.

“The argument that immigrants pose a threat to the cultural underpinnings of a free country such as the United States has been made by respected scholars and is shared by many well-meaning people, who deserve a thorough response,” writes Mr. Vargas Llosa. He provides one in the book, which explains which type of people leave their homelands, why they do so and how best to assess their impact on our economy, culture and politics.

To find its pro-immigration soul, Mr. Vargas Llosa argues, the party of the right need look no further than Ronald Reagan. The Gipper today is as popular as ever in the GOP, except for when the topic turns to immigration. Reagan signed the 1986 amnesty, mocked “the illegal alien fuss,” championed guest-worker programs and frowned on barricading the southern border. Mr. Vargas Llosa, a veteran journalist and senior fellow at the free-market Independent Institute, explains why the same president who won the Cold War was right about immigration, too.

Since the beginnings of the republic, those who came first have felt unease about those who followed. The English, Scottish, Dutch and Germans wanted to keep out the Irish and later those who came from Southern and Eastern Europe. Some of the descendants of these groups now want to slam the Golden Door on Latinos. Thomas Jefferson and Benjamin Franklin both expressed nativist sentiments. John Adams even opposed high-skill immigrants, arguing that French economist Pierre Samuel du Pont de Nemours should be denied entry because America “had too many French philosophers already.” Du Pont ultimately immigrated to Delaware in 1799, and his son, chemist E.I. du Pont, would go on to start one of the most successful business dynasties in the world.

In our time, America’s nativist strain found expression in Samuel Huntington’s 2004 best seller, “Who Are We?” The late Harvard political scientist updated the “past immigrants were good, current immigrants are bad” argument for the 21st century, writing that immigrants from Europe “modified and enriched America” but that the post-1965 immigration wave, most of which is from Latin America, “poses a fundamental question: will the United States remain a country with a single national language and a core Anglo-Protestant culture?”

Mr. Vargas Llosa is having none of this, and Global Crossings presents considerable evidence to counter the claims that America isn’t absorbing new immigrants as it absorbed old ones. Latino immigrants are assimilating just as past groups did, he argues, even though their progress is sometimes difficult to detect because Latino immigration is continuing. Longitudinal assimilation studies, which figure in the time spent in the new country, show that subsequent generations of Latino immigrants are in fact learning English, increasing their education levels and climbing the socio-economic ladder. Almost all native-born children speak English, and only about one-third of third- and fourth-generation immigrants can still speak the language of their grandparents. As with previous groups, the second-generation immigrants outearn their parents and are better educated, according to census data. Latinos are also assimilating through marriage: A study from the mid-1990s found that, while only around 8% of first-generation Latino females had married outside their group, the number climbs to 26% in the second generation and 33% in the third.

Mr. Vargas Llosa’s primary focus is U.S. immigration policy, but he uses international comparisons to show how immigration has benefited other countries as well. The literature is replete with examples—the Lebanese in West Africa, the Chinese in Southeast Asia, the Moroccans in Spain—of poor people moving to a new nation and making it more prosperous. “Everywhere, immigrant workers willing to work cheaply help natives to trade up,” he writes. And just like the U.S. today, other countries periodically contemplate granting amnesty to significant numbers of illegal residents. Amnesty, he says, merely amounts to “the acceptance by host countries of their failure to match the law with reality, and to establish policies that have long-term horizons and do not need to be constantly overhauled.”

Immigrants are drawn, first and foremost, to economic opportunity in the U.S., writes Mr. Vargas Llosa. Thus the issue isn’t whether we should grant amnesties or enforce the law. “The real debate,” he says, “is between accepting and negating reality. The root of the problem is that too many foreigners have been chasing too few visas.” Mr. Vargas Llosa says that our policy makers should focus on keeping the right incentives in place to attract and integrate immigrants rather than stanching the flow. “Immigration is not a threat to culture, the economy or security,” he concludes. “It is, pure and simple, the right to move, live, work, and die in a different place to that in which one was born—the victory of choice over chance.”