Cost is a
very rough indicator of footprint.
A modest e-book reader runs about $200. That's the cost of about 3 cases of 20# letter-sized paper (10 reams/case), or 15,000 sheets of paper.
At ~300 pages/book, that's about 50 books cost equivalent.
Note that printing output tends to run higher, at $0.01 -- $0.25 per page (higher for colour inkjet and laserjet).
Depending on ones reading patterns, the cost (and environmental impacts) of an ebook reader could well come out ahead. It's also generally easier to carry such a device than the equivalent number of printed books or documents.