According to a study by the Cleantech Group, you need to download 23 books for the Kindle before it breaks even in CO2 use.

Also, as pointed out in the comments, Kindle versions are $9.99 compared to maybe $15 for a new book, but you need to add $299 to the price of the device. At that rate, you have to buy a Kindle and 60 books ($900) before the Kindle’s prices win out.