Don't think quite what you're looking for exists—although Child's own commentary isn't wholly terrible, and there is plenty of individual scholarship to trawl through. (I would also recommend Mary Ellen Brown's Child's Unfinished Masterpiece for an analysis of Child's compilation process.) In general, I'd advise taking any source that seeks to connect these songs to anything "ancient" / "pre-Christian" with a grain of salt—this is a Victorian/Edwardian fantasy with a bizarre amount of staying power.