A mass-produced piece of plastic and electronics more environmentally-friendly than a simple book? Possibly, at least according to a new study released by the Cleantech Group. While the group found that the Kindle's upfront environmental impact was indeed fairly significant, they also found that the numbers can change dramatically over the course of the device's lifecycle -- depending largely on the users' reading habits, of course. More specifically, they say that the Kindle can produce a potential savings of 1,074 kg of CO2 if it replaces three books a month for four years, or a whopping 26,098 kg of CO2 if the Kindle DX is used to its fullest capacity. They also found that the Kindle would still break even if it replaced just 22.5 books over its lifespan, although they're quick to point out that its impact can turn to a negative if folks continue to buy books and print periodicals in addition to e-books and don't recycle them.