The family history detail you're chasing might be hiding in an old history or genealogy book.

Here's a quick listing of books that can be found online for free:

• Digital Public Library of America 
• FamilySearch Digital Library
• Google Books
• Hathitrust
• Internet Archive

Here are other (less-frequently-used) sources :

• Annas-archive.org
• Gutenberg.org
• Libbyapp.com
• Librivox.org  (audiobooks only)