The works on this page are taken from the SF Masterworks and Fantasy Masterworks series and from David Pringle's Science Fiction: The 100 Best Novels and Modern Fantasy: The 100 Best Novels. The reason for using these lists (as opposed to e.g. the Hugo and Nebula Awards for Best Novel) is because they make a clear distinction about which works are science fiction and which ones are fantasy.
In order to appear on this page, a work must be on the American, Australian, or Canadian public domain. The reason for this is that these countries are the only anglophone countries with active Gutenberg Projects. Furthermore, in order to appear on this page a work must be uploaded to one or more of the following: Project Gutenberg, Project Gutenberg Australia, Project Gutenberg Canada, Faded Page, Roy Glashan's Library, or eBooks@Adelaide.
American copyright law protects works for 95 years from the year of first publication, meaning that works published in or before 1930 are in the American public domain while works published in or after 1931 are still under American copyright. Australian copyright law protects works for 70 years from the year of the author, meaning that works whose author died in or before 1955 are in the Canadian public domain while works whose author died in or after 1956 are protected by Australian copyright. The Canadian public domain is currently frozen until 2043; until then, works from authors who died in or before 1971 are in the Canadian public domain while works from authors who died in or after 1972 are under Canadian copyright.