Alex Ross has written a book that deals with the last century of music from many different angles: political, social, personal. If you know very little about modern classical music, this text introduces you to the important composers and pieces as well as why they are considered so highly in a way that is enjoyable to read and makes you want to hear the pieces as he discusses them. The approach to the music is very complex but never looses sight of the works themselves and how it feels to listen to them. The author also has provided samples
of many of the pieces he discusses for free on his The Rest is Noise blog. Be careful, this book takes a long time to read as you will keep getting up to listen to new pieces and getting lost in music you never knew you would love.