As a retired philosopher of religion, I find this book appealing--especially its emphasis on the glorious, soaring, wordless theme with which the Ring ends; and also its finding parallels in the Judgment theme in Mozart's Don Giovanni and in Shakespeare's Cordelia in King Lear. Following Nietzsche, however, the authors reject Wagner's last opera, Parsifal, and apparently all religion--Christian, Buddhist, etc. Strange that philosophers that can grasp the meaning in a mytho-poetic work like the Ring reduce religious tradition to simple, literalist fundamentalism!
The Ring, according to the authors, was written under the influence of Feuerbach's secular humanist optimism, complicated by Wagner's own experience of the failure of worldly political utopianism. The ending symbolizes the "death of God," not merely the death of the Idols, as theologians would have it. The atheist pessimism of Schopenhauer came to seem more realistic to Wagner (with his last opera Parsifal??), but not before he finished the Ring with a ringing affirmation of life and love.
In trying to articulate how it is that, in spite of defeat and death, "not everything has been lost," they come surprisingly close, but are finally blocked by the ghost of logical positivism. In Mozart's Don Giovanni, the authors see Nietzsche's Ubermensch; a figure literally beyond good and evil, and not subject to any truth or negative judgment beyond the conflicting prejudices of finite creatures. In the judgment of the authors, the Commendatore, and the transcendent Judgment he symbolizes, is laughable. So also with positive judgment; the final theme of the Ring cannot be "redeemed by Love," but merely "triumph and vindication of Brunnhilde." (In whose eyes? If all judgment and truth are relative? If all who judge are temporary, finite, fallible creatures? If there is no Ideal Observer?)