A primary source in history is an artifact or document that is contemporary to the time period you are studying.
Search Library Search to find books by a particular person using the "Advanced Search" author field, e.g. " Voltaire, 1694-1778."
Try combining your search terms with some of the following keywords: sources, documents, correspondence, letters, diaries, etc.
Books published before 1926 are in the Public Domain, so the full-text of many historical books may be found via Google Book Search.
Project Gutenberg is a collection of over 50,000 e-texts in the Public Domain. Use the "Author" and "Title" searches to locate works by a particular person.