Kyle Gann is one of a vanishingly small group of people writing about and attempting to create a public for contemporary composition in the U.S. (He claims via e-mail to be the ONLY one!) This book is unique in covering developments over the past century exclusively in the States, and thus giving more coverage to lesser-known composers. For instance, I had never heard of Roger Reynolds before reading Gann's book, and he is now one of my favorites. (For books that place American developments in the context of developments in Europe, I recommend Morgan's Twentieth-Century Music and Griffiths' Modern Music and After, which begins after World War II.)
I used to read Gann's column in the Village Voice back in the late Eighties, but the scene he described sounded too dry and formalistic to pursue. Now I am finally catching up with contemporary "classical" music and can better appreciate Gann's contribution. His last section is on post-minimalist developments he groups together and calls "totalism." Sad to say, this music seems to have a "minimal" audience outside of small districts in NYC and perhaps the Bay Area, despite its attempt at populism, incorporating rock and intersecting with jazz as well in the so-called Downtown scene (John Zorn, the Knitting Factory, Tzadik Records).
Of course avant-garde music will probably always be marginal, and here's to Gann and all those who keep it alive!