I'm not a spa kind of guy, but Mata Chica has a new small spa.
The best spa in Belize by far is the one at the Lodge at Chaa Creek in Cayo. There is also one at Maruba on the Old Northern Highway (you'll probably like it if you are from California) and a poor one at Royal Mayan in Benque Viejo, and a couple of others in the works.
Belize books and publications by Lan Sluder:
Belize First Magazine http://www.belizefirst.com/
Fodor's Belize and Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize
Fodor's UpClose Central America
Belize Book of Lists 2000