There's a new report by the Cleantech Group that says that e-readers are green. In fact, according to the projected sales of e-readers in 2009 and 2012 could reduce co-2 emissions by 10.9 million tons.
Their analysis says that one year of use could offset the emissions made by manufacturing and distribution (and operation). Each following year of use could actually be preventing the release of 370 pounds of carbon dioxide per user (assuming individuals buy about 22.5 books a year).
The report mentions that using an e-paper based reader (currently
almost all of these are E Ink based) is best - because of the low power