George Williams' monumental The Radical Reformation has been an essential reference work for historians of early modern Europe, narrating in rich, interpretative detail the interconnected stories of radical groups operating at the margins of the mainline Reformation. In its scope--spanning all of Europe from Spain to Poland, from Denmark to Italy--and its erudition, The Radical Reformation is without peer. Now in paperback format, Williams' magnum opus should be considered for any university-level course on the Reformation.
I'm still working on this book. The adjectives that could best describe it are "exhaustive" and "encyclopedic." If I could take a time machine back and review this work in the 1950s (when the first edition came out), I would probably give it 5 stars. This book brings together the pieces to define the "Radical Reformation" (in contrast with the "Magisterial Reformation" of Luther and Calvin). Williams's argument is that the wide variety of dissident Christian religious groups that formed in Europe in the years after 1517 can fruitfully be treated as a whole. He makes his case well, providing more detail than is necessary (and sometimes losing the forest for the trees). His attention to detail is both the book's strength and its weakness. The detail makes his work valuable to researchers and teachers who want a complete guide to the issues of the time period, but it also makes it cumbersome for anyone who needs to grasp the "big picture" of what was going on religiously and socially at the time.
I am reading a recent edition of the work (originally published in the 1950s). It's clear, however, that most of the work has not had any significant revision; a quick consultation of bibliography and notes indicates that few if any references are to more recent scholarship have been added (even though there's been a lot of research in this area, much of it undoubtedly spurred by this very work). The focus is mainly on personalities ("great men" with a few "great women"), theological arguments, and institutional history, with more superficial treatment of social and economic issues. A welcome exception is the chapter on marriage and family (not in the original edition), which includes thoughtful discussion of these issues and relies on more current scholarship (1980s and even 1990s).
Overall, this would be a very useful reference book for a teacher, an undergraduate writing on a topic covered by the book, or a more advanced scholar just starting out in this field, especially since Williams cites editions of primary sources and/or quotes extensively if translations were not readily available at the time of publication. Most people, however, would probably not want to read it cover-to-cover. (An abridged version would be welcome for a more general audience.)
UPDATE: now that I have finished this, I would recommend that someone short of time read the summary of evidence in the final chapter (and read other chapters only if their interest is piqued by topics mentioned in the final chapter).
As can be noted from the dates of reading, I invested an extraordinary amount of time in this book, most of it feeling quite frustrated. Perhaps the most important thing to note before launching into a perusal of this enormous volume is the lead review on the back cover: "For more than 30 years, George Williams's monumental The Radical Reformation has been an essential reference work for historians of early modern Europe..." Ah, reference work. That, indeed, is how this book should be classified. There is an enormous amount of information on the Reformation's left wing from its precursors in the middle of the Fifteenth Century until the relative peace of the late Sixteenth Century. No player is too minor for Williams' scrutiny. No story too obscure. The unwary reader drowns in detail. Adding to the confusion is Williams' writing style which is prolix in the extreme. There are some chapters summarizing the grand theological movements of the time, which are helpful to gain some perspective on the deluge of details. Other than that, however, the book is best used as a reference work for the serious student of Luther's antagonists to his left and for those interested in digging out the historical underpinnings of antipedobaptism.