This is a foundational work that helps put the notion of the three worlds in context. My favorite parts of the book, and where Stewart seems warm up, is in the later chapters where he analyzes and interprets Scottish and Welsh ballads for their mystical content. It also includes the text for a Journey to the Underworld. I would love to find a recording with Stewart's music and voice. It seems that his website does not currently offer a recording (it was last available on cassette tape!)
If you ever get a chance to meet him in person or listen to him perform by all means, do! He is a master bard, storyteller and teacher.