I have found that one of the most important parts of studying composition is having a broad knowledge of repertoire, particularly in the twentieth and twenty-first centuries.
For beginners and those interested in expanding their listening tastes, I have found two books to be virtually indispensable; both are quite general, but give an excellent "way in" to works and composers from all periods of classical music.
The Rough Guide to Classical Music
Exactly what one would expect from the Rough Guide series, famous for their travel guides. Regularly updated, it tends to keep to the better-known composers, but does include some who are more off-the-beaten-track.
1001 Classical Albums You Must Hear Before You Die
Edited by eminent composer Julian Anderson, this is one of the most open-minded and comprehensive books on classical music to be found anywhere. Ranging from Hildegard of Bingen to Helmut Lachenmann, and Guillaume de Machaut to Philip Glass, every genre is covered in a positive and accessible manner. In addition to the 1001 albums of the title, each recommendation comes with further listening suggestions, meaning that there are almost 3000 recordings mentioned in total!
As a bonus, and as a general introduction to modern music, it is hard to find better than Michael Nyman's "Experimental Music". Perhaps a little outdated now, but not too challenging, giving more in-depth information about modern music than the two mentioned above. It does however assume some basic knowledge of the various genres of twentieth century music.
For more advanced composers, there are some famous but still relevant writings on music that should be explored.
A seminal work of one of the most radical composers of the last century, forcing the reader to redefine their very concept of "music". Includes transcripts of interviews and lectures.
Steve Reich: Writings on Music 1965-2000
Regardless of its laid-back approach, this is quite a technical read, but charts a development in music away from the high modernist thought of central Europe. Includes the famous essay, "Music as Gradual Process".
Theodor Adorno: Philosophy of Modern Music
Enthralling and infuriating in equal measure, Adorno's comments on art and the rebuilding of a tradition after the wars provides not only an insight into the music of his time, but also an integral building block in modernist musical thought thereafter.
Brian Ferneyhough: Collected Writings
Not bed-time reading by any means, but an invaluable insight into music that could so easily be overwhelming.
