The pilgrimage route to Compostela is graced with an exceptional witness from its early days: the Liber Sancti Jacobi or Book of Saint James. This book is found most famously in a twelfth-century manuscript from the library of the Cathedral of Santiago de Compostela, as well as in various other manuscripts. The text provides an encyclopedia on Saint James the Great and on the pilgrimage to Santiago de Compostela, the traditional site of his burial in Galicia in northwestern Spain. Of the five books included in the manuscript, Book I, the longest, contains the Sermons and Liturgy of Saint James. It is published here in English translation for the first time. It consists of two major sections: the first contains sermons and readings on Saint James, and the second presents a compendium of liturgical prose, poetry, and musical material suitable for the feasts of Saint James. Book I is accompanied here with the first full translation of the addenda that consists of material gathered subsequent to the compilation of the original five books and that was inserted at its end. The sermons and readings include works attributed to Pope Calixtus and Pope Leo, and others authored by Gregory the Great, Jerome, Bede, Eusebius, Augustine, and John Chrysostom. These sermons and readings stand as self-contained units that fit into the liturgies. The liturgical material is a compendium of readings and music for the feast days related to Saint James, including his Calling, Translatio, Passion, and Miracles. The musical pieces are the most extensive collection of musical materials dedicated to a single saint in the twelfth century. They include antiphons, responsorials, hymns, and processionals, with both older and original melodies. This book of the Liber Sancti Jacobi provides a picture of the power and importance of the saint, reflecting his significance and that of Santiago de Compostela, along with Rome and Jerusalem one of the three major Christian pilgrimage sites during the Middle Ages. 566 pages. Preface, introduction, notes, bibliography, index, and illustrations