This article deals with the ex-Capuchin, chaplain and later parson at the Brno parish church of St James, P. Maurus Simonis (*1740-†1815) and his catalogue of the manuscript library which came into being in the Middle Ages, was permanently maintained at the church and only in 1931 did it become a part of the Brno City Archives. The manuscripts – 125 codices – serve as valuable evidence of book culture in medieval Brno. The definitive catalogue was compiled in 1805, while its first "critical" version, which does not include all manuscripts, dates from 1802. On the evidence of numerous specimens, our paper analyses the way of describing of external features – watermarks, writing, decoration and binding – as well as the content of individual codices, and it compares the work of P. Simonis with the previously unofficial conclusions of the modern catalogue which is currently at the printers.