Experimental disco? Yes, the genre exists, and it was the specialty of the late Arthur Russell, who stood at the axis of underground sounds in early-1980s Manhattan. His eccentric output is mostly out of print, so these compilations are long overdue. ''World,'' an apt introduction to his singular fusion of disco, dub psychedelia, and avant-jazz unpredictability, grooves like club music for art majors. ''Calling,'' which culls unreleased solo cuts, is a more intimate work. Meditative tunes for when the party's over.