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-NoDerivs 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; for any purpose, even commercially. 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. If you remix, transform, or build upon the material, you may not distribute the modified material.
These books are available in MOBI format - good for Kindle.