This is a novel based on the true story of librarians with pack horses who delivered book to rural areas during the Great Depression.
From Booklist:
Cussy Mary Carter knows no one will want to marry her with her blue skin, a rare hereditary condition sometimes found in the mountains of Kentucky. She prefers it that way, since a married woman cannot work for the WPA, and being a Pack Horse librarian is the one light in her lonely, hardscrabble life.