ENThis article briefly presents Vilnius University professor of architecture, dean of the Faculty of Arts, J.Klos’ (1881-1933) life and his versatile activities in Vilnius. It offers a comprehensive study of the collection of his books left after his death, now being kept in Vilnius Academy of Arts library of rare publications. According to his books, notes and records in them, the author tries to characterize the architect, his interests, his viewpoint to architecture and aesthetic principles. J.Klos’ books reveals his utmost devotion to his profession and his- real profesionalism, as well as intelligence and industry. Dedications in the books, given to the professor, helped the author to learn and describe J. Klos’ relationships with other prominent figures of culture - J.Bułhak, J.Remer, S.Loza.