The world of e-book readers is in constant change these days, it would seem. This past month, two new e-readers were released to give Amazon’s Kindle a run for its money in the eco-friendly book space.

First up is Barnes and Noble’s new Nook, an e-reader with a 10-day battery life and weighing in at only 11.2 ounces. It comes with WiFi and AT&T 3G, a replaceable battery, PDF support, and has two screens—one for reading and the other (color) for easy browsing. It’s available now for about $260 (about the same as the Kindle) and can store 1,500+ eBooks, magazine, newspapers, and more. A fantastic green feature: you can “lend” your books to friends for up to 14 days!

Rumor has it that Apple will also soon be releasing their version of the e-reader with the Tablet (set to come out in 2010 according to BusinessWeek).

Using an e-reader saves energy in a number of ways:

• By reducing the number of books that are produced, we reduce deforestation, which in turn conserves carbon-absorbing trees.
• You’ll need less shelf space to store your books, which means you can presumably have a smaller home (less square footage to heat and cool).
• Buying books electronically saves on the fuel you’d use to go to the bookstore or the shipping require should you order a paper book online.

In fact, a report by the Cleantech Group has concluded that e-readers like the Kindle 2 can help you go green by saving 1,074 kg of CO2 annually (assuming you would avoid buying about 3 physical books/month).