Experimental Music: Cage and Beyond by Michael Nyman – this is a good overview of western music / musical figures (improvisational, composed music, modern-classical, minimalism, free-form, dadaism, Fluxus, et al) during the latter half of the 20th century.
Space is the Place: The Lives and Times of Sun Ra by John F. Szwed – interesting insight into Sun Ra’s life; brief overview of his youth, his ‘becoming’ Sun Ra, musical / life practices, events, philosophy etc.
Derek Bailey and the Story of Free Improvisation by Ben Watson – This is a really interesting account of the late 60s / early 70s (and beyond) scene / events surrounding players like Derek Bailey, Gavin Bryars, Anthony Braxton, Peter Brotzmann, Evan Parker, Han Bennink, Tony Oxley, et al) - Derek Bailey’s “Company”, Incus Records, etc… it’s engrossing.