Part of the answer will also vary by where you live. If you live in a major city where transportation is plentiful and there a lot of book stores which will be stocked and resupplied regardless of your purchasing then the carbon foot print of a single pBook is, comparitively small.
If, on the other hand you live somewhere remote, or where transport of physical product is not easy (Hawaii, one of the islands in Alaska) the foot print of a pBook will be much greater.
Regardless of actually figures and ignoring the ecological impact of the reader itself, for every eBook you purchase vs a pBook you are having a positive impact - which too me means I can tell my wife I'm saving the planet every time I'm reading and she wants me to do something else!!
