"Not an attempt to understand... Just an attention to the activity of sounds."
One of the great strengths of Nyman's short book is his careful attempt to define experimental music before he moves on to discuss the artists and their music. To summarize and paraphrase, he says experimental composers are excited by creating "a process of generating action," involving situations or fields delineated by compositional rules, but leaving them open to the performers. (4)
Experimental music is uncompromisingly radical, and represents an ongoing influence on creative music, but has certainly not become any sort of popular movement. So for instance, while the early "minimalists" Young and Riley were arguably part of the experimental tendency, as were Reich's early phase patterns, (and hence are included here by Nyman), the later works of Reich, and especially Glass, are no longer open and experimental. And while Eno and recent techno/ambient artists have been influenced, their innovations have been more technical than conceptual by comparison.
My recommendation if this sounds intriguing -- check out anything by the English free-improv group AMM, which is nowadays constituted by Eddie Prevost on percussion, Keith Rowe on guitar and electronics, and John Tilbury on piano!