A couple of good sources for information on proas -- i.e., single-outrigger multihulls:
Outrigger Sailing Canoes -- a blog by Gary Dierking, New Zealand-based American multihull builder, designer, and author of Building Outrigger Sailing Canoes.
ProaFile - a blog by proa designer Michael Schacht, a Seattle-based proa designer.
Both have lots of info. on both modern and traditional proas and occasionally on other types of multihulls. I know Gary Dierking and edited his book, and think it's very good.