http://www.larplist.com/mainlist.php?op=nalist - pretty decent list of larp groups, but has some outdated information.
http://larpwiki.mortalisrpg.com/index.php?title=Category:Larps - US specific list of larp groups. The rest of this site is pretty great as well.
http://www.larping.org/ - Videos, articles, info.
Leaving Mundania by Lizzie Stark - really interesting book covering a large variety of larps.
And check out some larp documentaries on youtube. It always helps to actually see it happening to understand it. Monster Camp and Realm of Larp are also pretty good.
I hope that helps! =)