The past twenty years have seen a great debate about copyright law. The catalyst was the Copyright Term Extension Act of 1998, which extended the term of existing copyrights for twenty additional years. This law was found to be constitutional by the U.S. Supreme Court in 2002 in Eldred v. Ashcroft. Lawrence Lessig, then a law professor at Stanford, wrote an influential book, Free Culture, that was critical of the lengthening of copyright terms and the expansion of copyright owners' rights. Lessig was one of a number of critics of American copyright law. On the other side is the novelist Mark Helprin, who in 2007 wrote an op-ed for The New York Times calling for authors to have a perpetual copyright to their works. The criticism he received prompted him to write Digital Barbarism, expanding on his Times piece to argue for increased copyright protection for authors. Below are some of many books discussing the implications of the copyright laws.