Here are some sites I've found to be exceptionally informative or interesting.
I had to special-order this pamphlet through my local library, some years ago. Now it's on the web.
Identifying Poison Ivy, Oak and Sumac
Axelrod Broadcast Solutions, LLC
Framingham, MA, USA
SBE Certified technical consulting, solving audio, video, radio frequency, and IT problems in cable, broadcast, commercial, and residential systems.