I currently believe that ebooks are far "greener" than paper books.
In the US (which is about 1/3 of the global book market, iirc), in 2006 1.5 million metric tons of paper were used to produce books, and only 5% or so of that was recycled materials. Of that, 1
billion books were unsold, and little of that was recycled. And about 70% of the source of paper for books -- and the industry's carbon footprint, coincidentally -- is in cutting endangered forests, rather than sustainably managed ones. Source:
http://www.ecolibris.net/book_industry_footprint.asp
The book industry could get greener by shifting to recycled materials or some type of rapid POD (thus reducing waste), and ebook impact hasn't been studied in-depth. But I expect that the environmental impact of storing and distributing ebooks is still lower than the impact of printing, warehousing and shipping paper books.