The English-language version of this book had a long genesis. Ultimately it goes back to the year 2000, to a Brussels colloquium "Musique contemporaine: théorie et philosophie" organized by ESCOM and led by Irène Deliège. The contributions to the colloquium and other essays were published in French in 2001.
Only in 2008 did the English version appear (the ebook in 2010), and it is the definitive form of the book because it incorporates five statements from contemporary composers on their aesthetic: Pierre Boulez, Brian Ferneyhough, Jonathan Harvey, Wolfgang Rihm and Helmut Lachenmann. Indeed, for readers like myself who are not academics, merely fans of 20th-century modernism and beyond, these chapters will probably prove the most interesting.
The interview with Boulez deals mainly with the composer's thoughts on Mallarmé and chance. The Lachenmann interview elucidates why this extremely avant-garde composer nonetheless feels that he is an heir to the classical tradition. Ferneyhough's interview is mainly on his opera "Shadowtime", while the Rihm interview delves into his expressionist way of writing and his a wide span of his career.
Instead of an interview, Jonathan Harvey is represented by a powerful essay of his own: "Music, Ambiguity, Buddhism". Harvey not only explains the aesthetic behind his pieces "Wheel of Emptiness", String Quartet No. 4 and "One Evening", but he also describes the play of ambiguity and spiritual resonances that he finds in Beethoven, Wagner, etc.
Alaistair Williams' "Postlude: Lachenmann, Rihm and the German Tradition" is also new to the English edition, helping to counterbalance the French-centered perspectives of the original publication. I suspect that this is simply an abridgement of the same author's Music in Germany since 1968.
What I've mentioned so far is in fact a small portion of the book. Most of the chapters are on more philosophical or technical issues: the legacy of Adorno, how serialism or free atonality can be reconciled with other styles drawing on the classical tradition, etc. Richard Toop contributes a passionate essay against any attempt at a "theory of New Complexity", since those particular composers are each doing something different from each other and are less theoretically-minded than commonly supposed.
I'll leave those chapters for academics working in the field to evaluate, but even for simple fans of the avant-garde, there's much to enjoy here.