Gina
Your caterpillar is one of the prominent caterpillars, Family Notodontidae. Since you have a Wagner book then you are into looking at caterpillars. So from my research I think it is either a Oligocentria semirufescens - Red-washed Prominent - http://bugguide.net/node/view/11752/bgimage OR Schizura unicornis - Unicorn Caterpillar Moth- http://bugguide.net/node/view/28015/bgimage?from=24
You have seen this from several angles so perhaps you can tell which one. As caterpillars get close to pupation, which yours may be, they sometimes show a little color difference...so look especially at the location and shape of the protuberances.
If you click on 'Info' on each of those websites there will be a description of the larvae to help figure out which one.
My guess is it is the Unicorn caterpillar, but yours is not as green as the other photos on the website. That could be because of lighting for the photo or a caterpillar ready to pupate.
Bugguide is a good source to help ID insects. Let us know what you think it is :-)
