The book by W.E.B. Du Bois is at Project Gutenberg (
http://www.gutenberg.org).
The others are recent books, published after 1923, so they are under copyright protection. Unless the author, or someone with the permission of whoever is running the author's estate, puts it online somewhere for free, you will probably not be able to find free versions.
Sites that scan copyrighted books and make them available with no payment to the author are inconsiderate to the author, and they're illegal. You would not want that to happen to your work.