SINCE the late '60s, architectural history has changed from a little- known pursuit of scholars to a minor national pastime. The grassroots turn to restoring old houses has certainly played a part, especially in the revitalization of urban neighborhoods. Economic advantages brought to investors by the 1976 tax reform act made historic architecture interesting to businessmen. Interiors have not attracted the same attention, and the result has been an imbalance in the reclamation of old buildings: facades are restored, while the insides, gutted, are replaced "contemporary." The process of interior extraction is so widespread that ours may be dubbed in retrospect the Gelded Age.

Peter Thornton, distinguished curatr at London's Victoria and Albert Museum, has produced a big and expensive volume with the title Authentic Decor, which may play a part in banishing recessed lighting, potted rubber plants and scrawny window-blinds from restored interiors. The author chronicles in words and pictures changing styles in interior decoration from the 17th century -- on which he is the recognized expert -- to the 1920s.

The documentary basis of the book is a collection of paintings and some photographs showing interiors furnished and in use when they were new. To this Thornton adds knowledge of his own gleaned from many years devoted to the study. "If we consider the rooms in which our own personal friends and acquaintances live," he writes, "it is obvious that there are many different ways of arranging and decorating a house, and this was always so. Nevertheless, each period of history has its own way of seeing things -- its own 'period eye,' as it were -- which, by somstrange process, seems to affect pretty well everyone."

It is this "period eye," the Zeitgeist, that Thornton seeks to define in its various forms over 300 years of living in houses, emphasizing Britain and Europe, with some attention also to the United States. For convenience he divides his material into 50-year segments, each beginning with a thoughtful essay rich in examples and illustrated by architectural drawings and various details such as mantelpieces or window hangings appropriate to the time in question. Following this are the interior views, each of them fully described according to its decoration.

The available documentary material fairly well restricts the book to upper-class houses and in many cases royal apartments, with a few views representing the middle class. This is a hazard of the pursuit. Lower-class private interiors were seldom drawn except in cartoons, and illustrations of modest interiors only begin to appear in any abundance with the 20th-century prolifertion of the snapshot. Amateur photos did not interest Thornton, who confesses that he always "chose the most charming picture available," thus basing his selections on the art historian's judgment rather than that of the historian.

Thornton's text is a feast of details about authentic lamps, chairs, beds, tables, curtains, carpeting and other elements in interior decoration. In his early chapters he covers the beginning, so to speak, when decoration of this sort was restricted more or less to the nobility. Interior decoration rose to popularity toward the end of the 18th century, causing the educated public to go in droves and knock on stately doors and beg to be allowed to look around. By the 1770s most great houses were open to the public and in some instances larger crowds made life there difficult. The widow who owned the sumptuous Hotel de Thelusson, a private house in Paris, was forced to issue tickets in an effort to control the flow of tourists through her salons.

THE outstanding feature of Authentic Decor is the several hundred documentary illustrations of rooms. Many are in color, handsomely reproduced -- indeed, the quality of the photographs is higher than that in any decorative arts book on the market. The material is strongest for the 18th and 19th centuries; for the years after about 1870, by comparison, it is rather sparse, with too heavy an emphasis in the modern years on rooms or settings decorated for exhibit instead of actual use as places to live.

The best of the rooms transcend interior decoration and suggest the lifestyles of those who lived there: A demure though brightly colored Copenhagen parlor of about 1814 has little in it -- sofa pushed back into an alcove, two straight chairs, two tiny half-moon tables, a cluster of pictures. Showing soldier- like order, it is as "clean" and "rational" as anything we call "modern." The same can be said of the Dutch study of about 1690, with bare, scrubbed floors and curtains that pull up like sails on a boat.

In Vienna in 1819 a family is drawn with its evening pastimes like moths to the artificial light of a central oil-fueled astral lamp, much as families today are drawn to the television. The innovative lamp, with its bright Argand burner (invented 1783) may have changed their habits even more than TV has changed ours.

Twenty years later what Thornton styles the "royal love nest" of King Friedrich Wilhelm III and Countess Liegnitz is a luxurious little sitting room that reflects its amorous role, done up elegantly in blue and white, the walls filled with favorite pictures, the sofa and chairs cushioned in silk.

The study of "an intellectual lady" in Victorian New York 40 years thereafter is delightfully personal, its decor inhibited by "historical" conceits about as remote from the comfortable, centrally heated life of the lady pictured as golden oak is from us, but sounding the ame ring of nostalgia.

Peter Thornton's book can teach much to architects and designers today and maybe help curb some of their abuse of the insides of old buildings. But after all, since the final responsibility for the structures lies with the clients, Authentic Decor may best serve the architecture buffs. This affectionate look into the souls of rooms will shake the faith of the most ardent proponents of "facadism."