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 OSI Approved License licensed ebooks are free to read - the people who created them want you to read and share them.
Open source licenses are licenses that comply with the Open Source Definition — in brief, they allow software to be freely used, modified, and shared. To be approved by the Open Source Initiative (also known as the OSI), a license must go through the Open Source Initiative's license review process.
These books are available in EPUB format - good for iBooks, Nook, Kobo.