Does Google Books increase or decrease the market for books?

The Supreme Court decides by not deciding, thus leaving the Second Circuit decision in place:

Last year, a unanimous three-judge panel of the United States Court of Appeals for the Second Circuit said that Google’s project was lawful and beneficial.
“The purpose of the copying is highly transformative, the public display of text is limited and the revelations do not provide a significant market substitute for the protected aspects of the originals,” Judge Pierre N. Leval, an authority on copyright law, wrote for the panel.  

An argument against:

Authors Guild president Roxana Robinson called the decision a “colossal loss” for authors, underlining the Guild’s continued belief that “authors should be compensated when their work is copied for commercial purposes”.
“The price of this short-term public benefit may well be the future vitality of American culture,” said executive director Mary Rasenberger. “Authors are already among the most poorly paid workers in America; if tomorrow’s authors cannot make a living from their work, only the independently wealthy or the subsidised will be able to pursue a career in writing, and America’s intellectual and artistic soul will be impoverished.”