"Statistics on downloads of electronic files and sales of printed books are tracked, and sales of the printed volumes have not decreased! Indeed, after the complimentary distribution of twenty-one titles in 2008 that had for many years only been available in print, sales of these titles increased by 7% compared with the previous two years."

http://oi.uchicago.edu/research/pubs/epi.html

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http://delicious.com/Klausgraf/monograph_open_access