THE PLAN OF ST. GALL is a colossus of a book. Lavishly printed in three enormous volumes and illustrated with some 1000 pictures (about a quarter of them in color), it weighs 20 pounds and costs $325. Only scholars are likely to recognize its subject, the plan of a monastery, which was never built, prepared around 820 A.D. and preserved today in St. Gall, Switzerland. But anyone interested in the Middle Ages will be captivated by the wide-ranging exposition set forth by Walter Horn (professor emeritus of art history at the University of California, Berkeley) and the evocative drawings by architect Ernest Born. The Plan of St. Gall is an encyclopedia of medieval life.

In the course of their inquiry, the authors have assembled most of what is known about medieval architecture, community planning, rural economy and Christian culture; in doing so, they focus new attention on a seminal period of European history, the age of Charlemagne.

Charlemagne fathered the Middle Ages. Although he was hailed at his coronation in the year 800 as "the great and pacific emperor of the Romans" and he presented himself as the continuator and restorer of Roman civilization, Charlemagne was a Frank and a Christian. He governed a largely transalpine empire (present-day France, Germany, Switzerland, Austria and North Italy) that just barely included the ancient capital; and though he was a restless warrior, he saw himself as a kind of Old Testament priest-king, a David whose task it was to organize his realm according to divine law. During Charlemagne's reign, bishops and abbots attained political and economic power unknown before, becoming the chief administrators of the imperial territories. The tone of society became distinctly religious.

Not surprisingly, then a plan for a monastery is a most eloquent witness to this period of social change. Some 650 monasteries flourished in Charlemagne's empire; they were rigidly structured communities submerged in a routine of prayer and other spiritual activities. One of Charlemagne's great goals was to bring these communities into harmony under a single code of behavior -- the sixth-century rule of St. Benedict. Among its other precepts, this rule ordered that monasteries be economically self-sufficient. As a consequence, the St. Gall plan includes, in addition to the church and cloistered residence for 77 monks, 50 other buildings -- schools, guest quarters, medical facilities, bath houses and privies, numerous agricultural structures and workrooms for cooking and manufacturing.

Ostensibly, the plan was for a monastery in St. Gall; in fact, it is a utopian design conceived without heed to the exigencies of a rugged site or local traditions. In character, the complex represented on the plan is not very different from a feudal estate. Because so little is known about domestic establishments in the Middle Ages, the St. Gall plan is a major document, not only of the history of monasticism and ninth-culture, but also of the social, political and religious history of the entire period.

What would a monastery built according to the plan have looked like? That is the first and primary question Horn and Born try to answer. Like a modern blueprint, the design consists entirely of floor plans; it provides measurements, but no elevations, no indications of materials or structures and few practical directions. To realize its buildings and the activities they were intended to house, the authors had to mine the history of art and architecture, medieval technology, economics and religion.

Providing what they term "historical justification" for every detail of their drawings and scale models, they bring together information from a dazzling spectrum of subjects -- including monastic reform, Icelandic houses, bloodletting, the history of measurement, beer brewing, medieval toilet paper, and even surveying techniques used in 18th-century America. The scholarship is exquisite; the arguments clear and compelling. To envision the fireplaces in the various houses of the St. Gall plan, for instance, the authors search Germanic sagas and arson laws. To evaluate the efficiency of the monks' wine cellar, they adduce the oldest known wooden barrel and documents pertaining to ninth-century inflation. And to recreate the facilities for processing grain, they trace the development of water power (which, like plant grafting and breeding, was stimulated by the monastic need to supply food to large numbers of men engaged in essentially non-productive activities). f

Much in the book is, perforce, speculative; and many of the conclusions will provoke as much disscussion as concurrence. But the authors leave no doubt about the overall significance of the St. Gall plan. They show that, like so many other things produced during the age of Charlemagne, the plan is an ingenious synthesis of Mediterranean and Germanic traditions. The vast church building, for example, was patterned after the Basilica of St. Peter in Rome but was laid out on a modular system that originated in northern barns. The orderly arrangement of dependent structures recalls a roman military camp but its square schematization has German sources. Even the corner fireplaces that appear for the first time in the plan result from a merger of north and south -- the German open hearth and the Roman hypocaust. In a chapter written with Carolyn Malone of Princeton University, the authors trace the plan's influence, arguing that the design served as a paradigm for monastic construction for several centuries. Though never built as originally conceived, the St. Gall plan was, nonetheless, one of the great successes of Charlemagne's time.