Libraries tend not to have genealogical records or data, but they can be an excellent source of information and history about the area, maps, photographs, etc.

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US
Directory of Genealogical Libraries in the US – Hundreds listed by state with contact information, many with hot links
Allen County (Ft. Wayne, IN) Public Library – Extensive collection of resources and databases for Indiana and the Midwest
Library of Congress – European Reading Room
Library of Congress – Polish Directories
Newberry Library – Chicago
Polish Museum of America Library – Chicago

GREAT BRITAIN
Polish Library – London

POLAND
Polish National Library – Warsaw
Kujawsko-Pomorska Digital Library – Toruń
Małopolska Digital Library – Kraków
Wielkopolska Digital Library – Poznań