Mark Twain considered copyright law a licence for publishers to steal from authors and their families and worked tirelessly as an unpaid lobbyist to get Congress to extend the term of copyright beyond the 42 years his own works were protected for. He saw publishers as pirates waiting for his works, which they had made no contribution to, to pass into the public domain, so they could print and sell their own versions, thereby stealing his property and depriving his family.
But he then came up with a scheme to thwart these pirates. When his copyrights expired he planned to publish new editions including extracts from his autobiography at the bottom of each page. He believed these new autobiographically enhanced editions of his works would attract another 42 years copyright protection and would kill the market for the re-printed public domain versions of the original works the 'pirate' publishers would produce.
A lovely article from the December 12, 1906, edition of the NYT explains. Twain may have got the idea from Walter Scott's commentaries - Scott published new editions of his old works with commentaries, after which the market for the works without the commentaries reportedly shrunk drastically.