How Mexicans See the U.S. and Trump

If Paul Theroux’s new book on Mexico is a commercial success, he’ll have Donald Trump to thank for it. But the initial inspiration came from a young man who worked in a doctor’s office.

In 2014 Mr. Theroux visited a clinic in this Cape Cod town, where he spends his summers. The assistant who registered him made an instant and irksome impression. “ ‘Take a seat, Paul,” Mr. Theroux quotes him. “ ‘Fill in these forms, Paul. The doctor will see you shortly, Paul.’ It was Paul, Paul, Paul.”

Mr....