No one should buy an electronic book because he or she thinks they may save forests. There are more than sufficient forest plantations on this planet.
I just googled this, and learned that the US has the largest area of productive forest plantations in the world, many of which exist solely from the paper industry.
Needless to say this industry would suffer immensely, causing thousands and thousands of people to loose their jobs world wide. I can understand the other reasons to buy an electronic book, but not to save the forests.

Just wanted to share another view point.