There is much to be grateful for in the Brepols series Corpus Christianorum in Translation, but this volume surely should rank high on the list, since it brings an important part of a difficult medieval Latin text, the Rationale divinorum officiorum of William Durand of Mende, into an accessible English translation. Like many volumes in this series, it is translated by one of the editors of the Corpus Christianorum Latin edition of the same text.  Timothy Thibodeau has, in fact, dedicated his career to this liturgical commentary, and has published English translations of two other parts: the Prologue and Book I, and Books II-III.  Now almost half of his huge and complex work, three full volumes in the CCCM, edited from some 200 medieval manuscripts in two redactions (22-23) is available in an English translation from the critical edition. This volume, on the Mass itself, will be a special boon for the many students of medieval Europe who wish to know more about the medieval liturgy.
William Durand (c.1230-1296), Bishop of Mende in Provence, is known primarily for his Speculum iudiciale, an encyclopedic treatise on secular and ecclesiastical law written between 1271 and 1291. Durand's Rationale was probably written in Italy before 1286. It discusses the liturgical practices of the Latin Church, using "Divine Office" as a generic term for all liturgies (19), citing Isidore of Seville to explain that the word "office" (officium) means "efficacious" (efficium), IV.5.4 (87, note b). Book IV, subtitled On the Mass and Each Action Pertaining to it, gives a detailed description of the Mass as celebrated in the thirteenth century, including objects used, words recited, and actions taken by the priest. In the prologue to Book IV, Durand describes the Mass as broken down into 4 parts: Supplications, Prayers, Intercessions, and Praises or Thanksgivings, IV.1 (73), and then proceeds to describe each of these parts. This first-hand description of the actual practice of the thirteenth-century Latin liturgy will in itself be of interest to medievalists; but what makes Durand's work even more interesting is the fact that he gives an interpretation for almost every object, utterance and action.
Often these interpretations are allegorical, for example, at IV.6.1, the altar signifies Christ (91); at IV.34.3, Durand tells us that the Sanctus says "Holy" three times to indicate the Trinity, but "Lord of Hosts" only once to signify God's unity (286); and at IV.29.4, he explains that the altar cloth is called the Corporal because it signifies the shroud of Christ, and is folded four times lengthwise to indicate the four theological virtues, and three times in width to recall the three theological virtues (242). The allegories generally follow the lead of the Christological interpretations of medieval Latin biblical exegesis (Cassian's allegorical sense). For example, the movements of the priest follow the movements of Christ: the priest rises for the Gospel because Christ went up to Galilee to preach the Kingdom, he goes first to the left side of the altar because Christ came to call sinners, IV.23 (191), and stands in the middle of the altar to consecrate because Christ mediated between God and men, IV.13 (131). Often, as in allegorical exegesis of the Bible, the explanations allude to fulfillment of an Old Testament passage or usage: generally, the Eucharist follows Baptism as the manna from heaven followed passing through the Red Sea, IV.41.3 (331); more specifically, at IV.6.5 we learn that there are two candelabra at the altar because they signify the Law and the Prophets, and because fire is required for sacrifices as mandated by Leviticus 6:12 (93). At IV.1 Durand says that the priest makes his ablutions before the Mass to follow Psalm 25, "I will wash my hands in innocence" (79). But sometimes the interpretation is purely functional, such as the ablution after the Eucharist, which we learn at IV.55.1 is done so that no particles will be carried from the altar (475).
Some parts of the Rationale IV give a glimpse of the heightened sense of the sacred that accompanied the medieval performance of the Mass, such as the passage in IV.41-42, where Durand discusses why the Eucharist uses bread and wine, (342-343), even if this means that there is Blood without Body and Body without Blood, (350-351); what to do if the Blood spills, (369); proper penances for clumsy priests, (383); and, at IV.41.31, even the famous worry of what it means if a mouse nibbles the Host, (342-343). There are also purely theological passages, such as an interpretation of the Lord's Prayer at IV.47.7-11, where the seven petitions of the Our Father are shown to correlate with the seven last words of Jesus on the cross, the seven gifts of the Holy Spirit, and the seven virtues (417-419).
Thibodeau's introduction and notes also tell the reader about the author's sources and general knowledge. Durand was greatly influenced by the late twelfth-century Mass commentary of Lothario of Segni (Pope Innocent III) (26-27), although the lengthy commentary on the fraction of the Host at IV.51.3-13 (442-448) is in his own words (29). Durand gives several unlikely interpretations of Greek words: "Kyrie eleison" at IV.12 (127), "symbolum" at IV.25.6 (218), suggesting that he knew very little Greek. On the other hand, his lengthy explanation of the various Creeds in IV.25.6-30 (218-231), including a spirited defense of the filioque at IV.25.11-12 gives us a fascinating eye-witness glimpse into the Second Council of Lyons (May-July, 1274), which Durand attended as a peritus for Pope Gregory X (221, note c).
This translation will serve as an excellent introduction to a medieval understanding of the proper way to celebrate the Latin Mass and how that rite was understood historically and theologically. It could well be put to use for examples in the classroom, since even undergraduates will appreciate the rich textures of the interpretation. Furthermore, since the translation will spread knowledge about the existence of this text, it should encourage some tantalizing comparative studies with medieval biblical exegesis and literature. Finally, since the Rationale was translated into both French and German by the end of the fourteenth century (23-24), it will also be welcomed by scholars of medieval vernacular traditions, who may well have been longing for such a glimpse into the liturgical life of their authors.
 Guillelmus Duranti, Rationale divinorum officiorum I-IV; V-VI; VII-VIII, Corpus Christianorum, Continuatio Mediaevalis 140, 140A, 140B, ed. Anselme Davril, O.S.B. and Timothy M. Thibodeau (Turnhout: Brepols, 1995; 1998; 2000).
 William Durand's Rationale divinorum officiorum. A New Translation of the Prologue and Book One, Records of Western Civilization (New York: Columbia University Press, 2007), and William Durand of Mende: On the Clerical Orders and the Liturgical Vestments (Scranton/Chicago: University of Scranton Press, 2009).