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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 a sub-section in a four-part test that courts use to look at fair use. But in the case of the Google book scanning case, the term will carry extra significance because one of the judges overseeing the case wrote a famous article that helped define “transformative” in the first place.

The judge, Pierre Leval, was part of a three-judge appeals court panel that earlier this year stopped a class action against Google and ordered the lower New York court to first address the fair use question.

The appeals court ruling reversed a string of losses in the case for Google, which included a high-profile ruling by U.S. District Judge Denny Chin in 2011 that rejected a controversial three-way settlement between Google, the Authors Guild and publishers. (Judge Chin, who now sits on the appeals court alongside Judge Leval, is still hearing the original case by special assignment.)

The Authors Guild, meanwhile, filed its own brief that blasts Google’s transformative argument: “[The] only thing ‘transformative’ about Google’s display of snippets of in-print books is that it transforms online browsers of book retailers to online users of Google’s search engine.”

At the same time, both sides are invoking the publishing practices of Amazon(s amzn) to support their position on fair use: Google notes that Amazon’s Book Search pages can lead to a sale for the author on Amazon.com, and points out that Amazon’s own “Search Inside the Book” feature displays entire pages of a book. The Authors Guild, meanwhile, claims that Google Book Search pulls away customers who would otherwise buy books on Amazon.

Both sides’ decision to invoke Amazon is ironic, given that the retailers’ dominance in the book market was prime reason that Google and the Authors Guild decided to create the now-failed settlement in the first place.

In the latest filings, the sides also spar over whether the digital library created by Google is safe from hackers.

Here’s Google’s latest filing with some of the key parts underlined. (For more background, see my The Battle for the Books: Inside Google’s Gambit to Create the World’s Biggest Library. It’s available for $2.99 here.)

Google Opposition to Authors Guild Motion for Summary Judgment (1)

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