Frassetto's handy review of five of the major medieval heretics is written for an undergraduate course on the Middle Ages or for the curious general reader. In short, it is a popularization of scholarly works, many published before 1980 and some more recently, all available in English.
The insertion of the word "Great" in the American title indicates something of the slant of the work, a kind of careful admiration for the heretics whom Frassetto has chosen to illustrate heresies from the tenth to the fifteenth centuries: Pop Bogomil and Cosmas the Presbyter; Stephen and Lisois; Henry the Monk; Valdes (or Waldo) and the Waldenses; Raymond of Toulouse, Pierre Autier, and the Cathars; Fra Dolcino and the Apostolici; Marguerite Porete and the Beguines; Wyclif and the Lollards; Hus and the Bohemians.
The book is quite readable and basically informative. Frassetto has chosen his English sources well: Christian Dualist Heresies in the Byzantine World, c. 650-1405 provides 35 of the 40 end-notes citations regarding Bogomil, Heresies of the High Middle Ages is the main source for chapters 2, 3 and 4, Malcolm Lambert's The Cathars does the same for chapter 6. Quotations are from scholarly books and collections, not from primary material except in the case of Bernard Gui's Manuel de l'Inquisiteur, a 1926 translation of the fourteenth century Latin original.
In some places, a better knowledge of theology would be helpful, for example with regard to Wyclif's challenge to transubstantiation, Frassetto writes: "For Wyclif, this (transubstantiation) could not stand from a philosophical perspective because the bread and wine had to preserve their substance even if they were—in philosophical terms—only accidents." (170) That is a contradiction of the kind that Wyclif argued against! Accidents must inhere in a substance; they cannot exist otherwise since the definition of an accident is that which inheres in another and not in itself. So Wyclif argued that the substance of the bread and wine must remain to support their accidents, but that at the consecration, the substance of the body and blood of Christ is also present and received with the bread and wine. That is the doctrine that scholars compare to Luther's doctrine although the two are not identical and should not be confused, nor does Frassetto confuse them, but neither does he explain either accurately.
Frassetto seems to have picked up uncritically generalizations regarding mysticism and heresy. Without seeming to know well the works of Bernard of Clairvaux or Hildegard of Bingen, Frassetto notes regarding Margarite Porete that "the mystical approach was not unheard of throughout the Middle Ages and was advocated by many others who were deemed orthodox, including Hildegard of Bingen and Bernard of Clairvaux . . . . "(201) That is using too broad a brush! Other conclusions are better grounded and widely accepted: the influence of Wyclif and Hus on the Reformation for instance.
Apart from these concerns, Frassetto's book will prove useful and may be given to students who will enjoy both the slant and the stories of these medieval figures who illustrate a side of church and society that is too often disregarded and deserves more careful consideration.
Frassetto provides a Chronology, a Bibliography, and an Index.