Currently, the environmental impact of an e-book is only incrementally better than a printed book.
Creating a printed book is surprisingly cheap, environmentally. The materials are increasingly environmentally friendly, if only so that a single book edition can complies with the strictest environmental regulations among all the jurisdictions in which the book is sold.
And e-books can be surprisingly expensive, environmentally. Making and disposing of electronic devices uses a great deal of energy and generates a lot of pollution. If the electricity that runs the servers that store and deliver the books to you, and that powers and recharges your devices, is all from coal-powered electricity plants, for instance, the environmental impact of e-books may approach that of a printed book.
The trick, going forward, is to “green” manufacturing, recycling, disposal, and energy generation for IT and for the home. It’s possible for every step in the chain of creating, delivering, and reading an e-book to be powered by green energy. As this begins to happen, e-books will develop a growing, and permanent, sustainability advantage over printed books.