"[...] electronic music was soon set on a path apart from other music, to become a sphere (too often regarded as a secondary sphere) with its own institutions and proponents." (p. 18, emphasis mine)Griffiths doesn't reveal the identity of those who regard EAM as a "secondary sphere" (nor what he means by "secondary"), but he certainly seems to belong to them, since he mentions almost no composers that have devoted themselves to EAM.
From the book's index I've estimated that, of the 10-11 most mentioned composers, Stockhausen receives by far the most attention, followed by Boulez, Cage and Nono, and then Berio, Messiaen, Ligeti, Stravinsky, Babbitt, Kagel and Lachenmann. Surprisingly, Xenakis is not among these 11, and ranks below e.g. Elliott Carter, Maxwell Davies, Ferneyhough, Henze, Kurtág, Reich, and Scelsi.
The composers Griffiths discusses in connexion with EAM are almost the same as the ones listed above. Of those that's mentioned more than twice, Stockhausen again receives the most attention, followed by Nono, Berio, Cage, Babbitt, Boulez, Chowning and Schaeffer. These composers were obviously important for the development of EAM, but many of them wrote mainly instrumental and vocal music.
Many of the other EAM composers discussed in the book are associated with either IRCAM or computer music, such as Jonathan Harvey, Jean-Claude Risset and Charles Dodge. Xenakis' EAM isn't mentioned, and also left out are the entire French/Canadian acousmatic and soundscape traditions, e.g. Bayle, Dhomont, Ferrari, Parmegiani, Radigue, Schafer, Smalley, Truax, and Westerkamp.
Of course, Modern music and after is an introductory text, and numerous composers have to be excluded (for another perspective on this, see James Primosch's blog post Whose “Modern Music” and whose “After”?). I still think that Griffiths' text is a good introduction to the main trends in 20th century modernism, and a glance through Music in the late twentieth century, the fifth volume of Richard Taruskin's Oxford history of Western music, suggests that it too suffers from the same distorted view of EAM.
But privileging EAM that emanates from serial and computer music is an unfortunate bias that carries with it notions of purity, control and academicism. It's no coincidence that David Metzer chooses Stockhausen's Gesang der Jünglinge and Harvey's Mortuos plango, vivos voco as his prime examples of purity in Musical modernism at the turn of the twenty-first century.
Not all computer music sucks, and there is certainly boring and academic-sounding acousmatic EAM, but that's no reason for musicologists to neglect the latter.