If you are looking for a book or a document that is out of copyright (older than 75 years), there is a good chance you will find it on one of these sites:
Hathi Trust
On this page, you will find some examples of the types of web sites good for historical research. Many national libraries, historical societies, and museums, now offer extensive digital collections. Other web sites are specialized in specific historical periods or topics. These are just a few examples; thousands of amazing sites are available. Go to the section "GOOGLE SEARCHING TIPS" to learn how to find good web sites on your topic using Google.
General
Best of History Web Sites (Organized by periods and topics)
The Labyrinth (Resources for Medieval Studies)
Cold War International History Project
American History
From Revolution to Reconstruction
Densho Digital Archive (Japanese American Internment Camps)
Valley of the Shadows: Two Communities in Civil War America
Anti-War and Radical History Project
Immigration to the United States 1789-1930
European History
French Revolution Digital Archive
Liberty, Equality, Fraternity: Exploring the French Revolution
Seventeen Moments in Soviet History
Africa:
Transatlantic Slave Trade Database
Asia:
Historical Photographs of China
A: AUTHORITY
There so much questionable information on the web. Always make sure to check who is responsible for the information on a web page. Check for "About Us" or "Who We Are". If you cannot figure out who is behind the page, don't use the information!
B: BIAS
There's plenty of opinions about who did what and why. You might be looking for a particular bias. Just make sure it's the right one
C: CURRENCY
Our knowledge of history changes with new research, new viewpoints. Make sure your research has current information.
FoodTimeline.org
Find out what people were eating in the 14th century, or discover when potato chips were invented!
Fashion Era
Explore fashion history and costumes through the ages