The vast majority of trees which are cut for paper pulp are quick-growing loblolly pines which will be re-planted almost immediately, larger, older, nicer trees are usually cut for lumber, so one should be able to let the 8.85 pounds figure stand for paper products w/o concern for deforestation.
Here's a page which indicates most CO_2 production is for energy:
http://www.eia.doe.gov/oiaf/1605/ggrpt/carbon.html
And here's a page which indicates that CO_2 production is a much larger problem for the manufacturing of electronics:
http://www.energybulletin.net/node/49730
w/ a ratio of 12 to 1 for energy usage to weight, so my PRS-505 weighs roughly 9 ozs., so presumably required 108 ounces of fuel to manufacture (on-going energy usage is not considered)
http://www.epa.gov/oms/climate/420f05001.htm
gives us a figure of 19.4 pounds of CO_2 per gallon of gasoline which equals roughly 16.36875 pounds of CO_2 to make the ebook reader.
So getting two books for the Sony should make it roughly break even, and each printed book beyond that which is not purchased should result in a net reduction of CO_2 emissions, since the energybulletin.net page indicates that the embodied energy usage for electronics is much greater than the lifetime usage.
William