Share 'What are some great references for what a PLN is?'

I'm writing an article for a trade journal and was wondering if anyone has any good references to some articles about creating a PLN? or about communities of practice or anything along the history of PLNs in general.  I've done a few preliminary searches on the web and have not been able to find a whole lot.  Maybe I'm just looking in the wrong places.  Anyway, any help would be much appreciated.…