The Reformation of the Twelfth Century. By Giles Constable. (New York: Cambridge University Press. 1996. Pp. xx, 411. $64.95.) It is commonly said, whether true or not, that mathematicians and physicists do their best work when young. Constable's book is a demonstration that historians can have a longer period of creativity.Years of study have enriched his understanding of the past.This book is provocatively titled Reformation, but in fact the most obvious reformation, that of the sixteenth century, plays no role in the analysis. Some writers in the late eleventh and twelfth century lamented that their world was confusing and divided.When they wrote about ending the confusion and division, either by restoring a golden past or by ushering in a better future, the words reformare/reformatio flowed easily from their pens. It is from these contemporary senses of the word that Constable chose the title for his book. Since his book on monastic tithes (1964) and his edition of Peter the Venerable's letters (1967), Constable has crisscrossed the twelfth century from many angles.This book is a rich synthesis of forty years' study of the religious life and its reform(s) during the years 1040-1160, although the author sometimes steps outside those chronological boundaries. The range of his reading in printed sources and manuscripts is prodigious: He has a very good memory or a very efficient filing system. In this book, the reader has the sense of overhearing a heated, untidy discussion/argument among members of an extended family. Constable has set himself the task of unscrambling the voices in that discussion. The six central chapters treat the variety of reformers, the types of reform, the rhetoric and reality of reform, and the spirituality of reform. Constable is sympathetic to the aspirations of twelfth-century monastic reformers, but he is hard-headed in his assessment of motives and outcomes. His treatment of the literary commonplaces (topol) of reform is illuminating and warns the reader against a simpleminded literalism.The book is packed with information and insight-on topics ranging from monastic architecture to monastic beards. A few paragraphs or pages summarize fairly and precisely whole monographs and scholarly disputes. Constable deftly handles traditional topics, such as the vocabulary of reform, the office of the monastic advocate, or the complexities of monastic exemption. But fresh perspectives, born of years of thought, are evident on many topics.Without fanfare, cliches which still influence the study of religious life are challenged or demolished; for example, that monastic wealth inevitably led to a decline in religious life or that twelfth-century Cluny was lax or corrupt. …