Google(s goog) and the Authors Guild’s eight-year legal fight over digital books is coming to a head once again, as both sides prepare to make their final case next month about whether Google’s scanning of more than 20 million library books is fair use under copyright law.

In documents filed in New York federal court this week, Google argues at length that the scanning is “transformative” — a legal concept that gained importance after the Supreme Court used it in 1994 to rule in favor of rappers 2LiveCrew, who had sampled the Roy Orbison song “Pretty Woman” without permission. Google argues:

“Google has copied no more than is necessary to achieve its transformative purpose and give rise to the social benefits of full-text search…Google improved on existing indices so substantially that its use was transformative.”

The “transformative” factor is not an automatic shield against copyright infringement. Instead, the term is just…