There are various articles that discuss the differences in environmental impact of e-readers and books. For example:
http://theconversation.edu.au/weighi...-of-books-8331
http://www.zdnet.com/are-e-readers-o...nt-7000001689/
http://green.blogs.nytimes.com/2009/...er-than-books/
Thus, e-readers have higher impacts on the environment given production, but this is offset by the fact that they can store lots of books.
There is one catch that hasn't been mentioned, though, and it's the point that e-readers don't last very long. After a few years, they malfunction or stop working and need to be replaced. Books last much longer.
Another is that they require various resources that might not be readily available in the long run, including rare-earth elements.
Third, they require much more infrastructure to be maintained, from computers and storage devices (and even networks) needed to back up or access information to electricity. (This is critical if the e-reader stops working or is lost and data must be recovered from back up sources.) Books, on the other hand, have been made for centuries, before the use of electricity, and do not require a lot of infrastructure.
Given the threat of a resource crunch (warnings from the IEA, Morgan Stanley, Lloyd's of London, the U.S. and German military forces, etc., of peak oil), environmental damage, and continuous economic problems due to increasing debt, rising food and oil prices, and unemployment, books will become more important. And with a lack of paper, only the works that are considered more important will be preserved.
With that, e-readers will be helpful only as backups or for convenience (e.g., something to read while one travels, something used to read works that are not that relevant, used to search for phrases in text quickly), but even then smaller computer notebooks will be much more helpful as they contain additional features, such as word processing, spread sheets, etc. They will not be able to replace books.