This paper explores the codicological features, contents and history of BL Harl. MS. 281, an anthology of writings on music theory copied by a single hand in the early fourteenth century, well known inter alia for including one of only two copies of the Ars musice of Johannes de Grocheio. We argue that the anthology was commissioned by Guy of Saint-Denis, a monk of that abbey, and that this Guy was responsible for numerous corrections throughout the manuscript, in particular to his own treatise, the Tractatus de tonis, which closes the anthology. The opening treatise in the anthology, three books attributed to Guido of Arezzo, constitutes a carefully edited collection of writings, supplemented with a range of other texts, including the Dialogus de musica of Pseudo Odo, and writings about Guido of Arezzo that may have been specially compiled by Guy of Saint-Denis. We argue that the compilation provides a comprehensive manual of writings on music theory, all of which influence Guy in one way or another. We propose that Guy may have commissioned and then corrected the manuscript so that it could serve students at the College of Saint-Denis, established in Paris in the thirteenth century to enable an elite group of monks to pursue university studies. We also explore the subsequent history of the manuscript, in particular how it came into the possession of Jean Gosselin (c. 1505–1604), Guardian of the French Royal Library from 1560 until his death. The manuscript indirectly throws light on the otherwise unknown whereabouts of the French Royal Library during the wars of religion.