keyword: United States -- Discovery and exploration – 0 free books.
These books do not seem to be included in the Directory of Open Access Books.
These Creative Commons Attribution-NonCommercial licensed ebooks are free to read - the people who created them want you to read and share them.
You are free to: copy and redistribute the material in any medium or format; remix, transform, and build upon the material; under the following terms: You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. You may not use the material for commercial purposes.
These books are available in EPUB format - good for Kindle, iBooks, Nook, Kobo.