Great idea! Gutenberg has several books by du Bois (e.g.,
The Souls of Black Folk):
http://www.gutenberg.org/browse/authors/d#a226
For your Sony Reader, you'd need to either import the TXT file from Gutenberg (which won't look very nice), find a better format somewhere else, or do the conversion (using, for example, Book Designer). But I think I'm just going to go ahead, if it's okay with you, and I'll convert this title right now and then upload it in an hour or so.
The works from the '50's won't be in the public domain so Gutenberg is unlikely to have them. For those, you'll need to look for what you can buy (e.g., Sony, etc.).