The book "Letter of Petrus Peregrinus on Magnet" (1269), written by Pierre de Maricourt, active in the 13th century, is one of the key works in the history of science. This letter reflects discoveries and studies of the phenomenon of magnetism, important for the development of science and technology. Petrus Peregrinus describes in detail the properties of a magnet, the interaction of magnets, and the effect of magnetic polarity on motion properties. This document became an important contribution to the understanding of the nature of magnetism and its application in practical use. The letter of Petrus Peregrinus is an important link in the history of science, which influenced the further development of physics and engineering.