The HathiTrust repository is a good place to search for primary sources that are available in full text.
If you know the title of a book printed before 1929, check for an electronic copy of it online. Three sites with digital collections are:
The books at these sites are usually (but not always) freely available in pdf.
Searchable collections of early printed books are available at the Text Creation Partnership. The Partnership provides free access to millions of page images from two important collections: Eighteenth Century Collections Online, and Evans Early American Imprints. Search each collection separately:
The Evans collection contains thousands of items published in America from 1470-1790 including books, pamphlets and broadsides.
Eighteenth Century Collections Online is a collection of book, pamphlet, and ephemera material published in England during the 18th century, along with thousands of books printed in America.
Older published materials may be freely available in The Internet Archive. Identify older books you want to see using bibliographies or reference lists from secondary sources, or from a search of library catalogs. Then search for the title(s) in The Internet Archive. You can read the texts online or download them.
The Digital Public Library of America brings together digital collections from hundreds of libraries across America. The database will lead you to freely available content: photographs, books, maps, news footage, oral histories, personal letters, museum objects, artwork, government documents, and much more.
The U.S. Library of Congress makes available thousands of documents, images, texts, audio and video recordings online. Use the search box at the top of the page to automatically filter results to those available online.