It is the most important humanities research library, the main archive of Polish writing and the state centre of bibliographic information about books.

[3] Literature and making those works accessible to the public receives a copy of every book published in Poland as legal deposit.

However, the Załuski collection was confiscated by troops of Russian tsarina Catherine II in the aftermath of the second Partition of Poland and sent to Saint Petersburg, where the books formed the mass of the Imperial Public Library on its formation in 1795.

[18] This caused the destruction of 80,000 early printed books, including priceless 16th–18th century Polonica, 26,000 manuscripts, 2,500 incunables, 100,000 drawings and engravings, 50,000 pieces of sheet music and theatre materials.

The library collections also include photographs and other iconographic documents, more than 101,000 atlases and maps, over 2,000,000 ephemera, as well as over 2,000,000 books and about 800,000 copies of journals from the 19th to 21st centuries.

Notable items in the collection include 151 leaves of the Codex Suprasliensis, which was inscribed in UNESCO's Memory of the World Programme Register in 2007 in recognition for its supranational and supraregional significance.

[23] The Catalogue of the Archbishops of Gniezno and Lives of the Bishops of Cracow by Jan Długosz is a 16th-century manuscript illuminated by Stanislaw Samostrzelnik between 1531 and 1535.

It also holds works from other important composers such as Józef Elsner, Karol Szymanowski, Grażyna Bacewicz, Witold Lutosławski and Krzysztof Komeda.