Malcolm Lambert's Medieval Heresy: Popular Movements from the Gregorian Reform to the Reformation is unquestionably an excellent book. I am only giving it four stars because I have not read a great deal in the area. Hence the book could have some serious drawbacks that I am not aware of.
Lambert's book was written to serve as a text book for a course on heresies during the middle age. Overall it does an outstanding job of explaining how the Cathar, Huss, Lollard, Waldensian and several other lesser heresies emerged, how they came to conflict with the Roman Catholic Church and subsequently how their followers migrated into the various Lutheran and Reform churches that appeared during the Reformation. It is very easy to see why the three editions of this work were ultimately produced.
Lambert sees the heresies occurring as a result of the spreading of literacy, the growth of the guild and merchant class, and finally the appearance of Bibles translated into various vernaculars (i.e. French, German, and English.) It was the small merchants and artisans who were most attracted to the heresies. They were very devote and not willing to trust the great issue of their eternal salvation to the clergy as long as the bible was available only in Latin could claim they alone were able preach the Lord's message. Although the members of the heresies all came from a distinct social class, Lambert goes to great efforts to explain that their heretical practices could not be interpreted as being part of a class struggle. Being fervent Christians they simply wanted control over their own salvation. Similarly the bishops who fought against them did not see themselves as suppressing a political revolution. They saw themselves simply as fighting the devil who was promoting false doctrines.
The various heresies challenged the Catholic Church on the very issues that the Lutherans and Calvinists would later raise. They questioned the legitimacy of a professional clergy, child baptism and the absolution of sins obtained through confession. They also believed in radical poverty for the Christian Church.
The middle heretical movements had no theologians of the stature of Calvin or Luther but this was not the reason why they were ultimately suppressed or driven underground. The Catholic Church prevailed against the heresies of the middle ages because they had the solid support of the judicial, police and military organizations of the day. When the nobility embraced the Calvinist and Lutheran religions during the renaissance, they brought with them armed forces and organizations that allowed the Reformation religions to triumph in many regions.
I note the disgruntled comments from most of the other Goodreads reviewers which are likely due to the fact that the books is very dense and that the author is highly erratic in the way that he introduces new individuals, terms and concepts. Ideally one would read this book as a university undergraduate reading one chapter per week and then discussing it with colleagues in the tutorial. The professor and tutorial leader would need to be very willing to explain the many muddled passages. If you are not undergraduate enrolled in such a course, you will at a minimum need to consult Wikipedia on a fairly regular basis as you read this book in order to follow the author's argument.
Aware that he is writing a text book, Lambert then does not try to make his book easy to read nor does he try to make the data fit into any clear thesis. Lambert makes it quite clear in his introduction, he defers to the acknowledged experts in the various areas covered in the book even when he is uncomfortable with the conclusions drawn by the experts. In other words, Lambert is presenting a survey of the best current academic work on medieval heresy not trying to present his interpretation. He insists on his point of view only in those areas where he considers himself to have deep knowledge; specifically Lambert speaks most authoritatively on the subjects of French Catharism, English Lollardism and the career of John Wycliff. In this regard, Lambert is being rigorous but his scruples give an uneven quality to the book.
Many readers will also be frustrated by Lambert's insistence that there is much about the medieval heresies that will never be known because the heretics who tended to be artisans, farmers and trades people who left little documentation. Thus much of what we know about the heretics comes from the reports of the inquisitors who certainly had a very hostile bias. The situation is completely different of course when writing about the reformation where the leading players such as Jean Calvin and Martin Luther were both highly articulate and prolific. In writing about the medieval heretics, the historian most often resort to informed speculation. An historian writing for the general public will do so with a confident tone and not trouble the reader with the limitations of the data. Lambert, however, chooses to make no effort to clarify the inherently obscure.
This book is rigorous, thorough and informative. It is well worth the considerable effort required to get through it.