It changed the way people thought about historic buildings in this country.
Here was a distinguished academic saying that understanding architecture was important, but not as important as understanding how people live in it.
His book introduced a completely new way of looking at important historic buildings - as social history rather than art history. The book has changed the way that buildings are shown to the public, whether Madam Tussaud's extraordinary weekend house party at Warwick Castle or English Heritage's own displays at Brodsworth House, Yorkshire. Both organisations have moved away from concentrating solely on art and historical technicalities to explain how people lived, worked, loved and died in these buildings.
A book like this is published once in a generation and it has spawned thousands of imitators. My own work on Tudor and Stuart royal palaces, for instance, takes court etiquette as its starting point, rather than the traditional way of the four orders of architecture.
Girouard's book still influences the way people write about architectural history and the way people view old buildings.