There are some things to recommend Stephen Johnson's introduction to Der Ring des Nibelungen, but the same critique that's often leveled against Der Ring itself applies to this commentary: it's awfully long. In fact, it's longer than Das Rheingold. But for the listener who's willing to devote more than two and a half hours to getting acclimated to the complex personal relationships, the convoluted narrative, and musical leitmotifs, this detailed commentary could be just the thing. Johnson offers an extensive introduction to the work's literary, musical, political, and philosophical genesis, as well as its subsequent cultural and political significance. That's all well and good, but for anyone who has some basic knowledge about Wagner, it's possible to skip directly to the commentary and cut over half an hour from the listening experience.
Johnson's commentary itself is clear and relatively concise, considering the complexity of the material, and he does a good job laying out the story, with accompanying musical examples. Unfortunately, but predictably, the examples come from the Naxos Ring with the Stuttgart State Opera conducted by Lothar Zagrosek, which has no continuity in the casting between the operas. This is one of the less successful Rings on disc -- the singing is at best undistinguished, but more often so distressingly wobbly that it's hard to tell what pitch is being aimed at. The orchestral playing is also sub-par; the anvils in the "Descent into Nibelheim" sound like they're played on pie tins. As an information source on The Ring, this commentary is useful as long as the listener keeps in mind that the illustrative performances generally fail to convey the music's true grandeur.