When you know the title of the item you are seeking, the first step is to search by title in an online catalog.
Reference books are useful for identifying people, places, events, dates, etc. Many articles in reference books include a bibliography of recommended sources. Here are just a few history reference books that may be useful.
The HathiTrust Digital Library provides access to a database of digitized books and other materials. You can search the catalog, search full text, or browse the public collections. Note that not all books are available full text.
To find only those books that are available full text, you can search the catalog on the main search page and click on the box next to "Full View Only."
To do a more complex search, click on the Advanced Catalog Search link. You can also select "Full View Only" on this page.
A second search engine, designed by Worldcat, is available here.
Google Books is a searchable database of books and magazine articles, but not all of the books are full text.
At the Basic Search page, you can simply type in keywords to do a search. Not all books in Google Books are available full text.
To search for only full text books:
Internet Archive is a database of full text books, magazines, audio and video files, maps, etc. It's a good place to find books that are old enough that they're no longer under copyright.
To search for a book, type in a search term in the search box on the home page and select "Texts" from the drop down search box.
If you want to do a more complex search, click on the Advanced Search link to the right of the search box on the home page. This allows you to customize your search in many ways.