Over time, he became increasingly involved in various dissident organizations. In December 1959, he became a vice president of the Union of Polish Writers ("Związek Literatów Polskich", ZLP). He also published in the magazine "Świat" (1951–1969). In 1962 he was the last president of the literary discussion society, Crooked Circle Club. In 1966 he was a vice president of the PEN Club. While in the late 1940s and 1950s he focused mostly on journalistic activity, later he turned to writing popular history in book format. In the 1960s he wrote his most famous works, historical books about history of Poland – the Kingdom of Poland in the times of the Piast dynasty, the Jagiellonian dynasty, and the era of elected kings (the Polish–Lithuanian Commonwealth). His book on Jagiellonian Poland was recognized as the best book of the year by the readers.