Bołdyrew, Aleksander. “The Letters of Priest Jan Piotrowski, „the One Friend at the court”, about the Army in the Military Camp of Psków in 1581”. Acta Universitatis Lodziensis. Folia Historica, no. 102, Dec. 2018, pp. 51-64, doi:10.18778/0208-6050.102.04.