There are several books in the Wildflower Center Bibliography that deal with edible plants in your state or region. Check the list below for some suggested titles.
Edible Wild Plants of Pennsylvania and New York by J. Tomikel
Edible Wild Plants of Pennsulvania and Neighboring States by Richard and Mary Lee Medve
Edible Wild Plants of Eastern North America by M. L. Fernald
Also, you might be interested in seeing a previous Mr. Smarty Plants question about edible fruits and plants in PA that Nan Hampton answered most thoroughly (including a list of edible trees and shrubs).