The worth of Polish Bibliography by Karol Estreicher was such important, that it should have been continued. Jan Muszkowski decided to do the continuation under the aegis of Warsaw Scientific Society. Firstly alone, then with the help from the team of librarians, he collected over 125 thousands of bibliographic descriptions of publications published in Poland between 1901 and 1925. The substance prepared for printing as bibliography covered letters from A to R burned down during Warsaw Uprising. However, his interest about bibliography were spreading more than „primary work” of collecting descriptions. Jan Muszkowski published a lot of articles and scientific works related with this issue. He edited also monthly paper called Książka and addition to Przewodnik Bibliograficzny. The name of Karol Estreicher appeared in his whole activity connected with bibliographic works.