(Boulez, for non-musical readers, is pretty much the leading figure — or at any rate the leading European figure — in the art music of our era [i.e. post-WWII]. For mathematical non-musical readers, an approximately equivalent person would be Jean-Pierre Serre.)
The interesting part of the site is, of course, the series of pages devoted to Boulez’s 1984 work Dérive I, which is described as:
an elegant, shimmering and vibrating eight minute work which explores harmony and texture from a chordal starting point using material which “derives” from three earlier pieces, Répons (1980), Messagesquisse (1976) and Éclat (1965).
(Obviously, “harmony” here is to be understood in the sense of vertical pitch collections — nothing to do with harmonic theory! I would probably have used the word “sonority” here instead.)
It seems the excerpts included comprise virtually the entire piece, so go have a listen. The commentary hardly constitutes a detailed analysis, but overall the quality is pretty good for something on the Internet.