Following on from a recent post, here's the International Journal of Community Currency Research. While there's not a huge amount in the journal, they also have a Yahoo Group which could prove interesting.
Thomas Greco is apparently popular with some of the alternative currency crowd, having written Money: Understanding and Creating Alternatives to Legal Tender (Amazon US, UK). I've also now noticed that Bernard Lietaer, the subject of the earlier post, has a couple of interesting sounding books out: The Ecology of Money (Amazon US) and The Future of Money (Amazon UK).
(Mostly via the Well's Future conference.)