I recommend starting with Glassworks
, his String Quartets
, as well as the piano suite Metamorphosis
. Together these will give you a survey of Glass's mature style, without much of the "I've heard it all before" aftertaste.
If you've been sold then, the next steps will be Symphony #5
, a gigantic work (actually an oratorio) which demonstrates how eloquent and flexible his minimalistic technique can be. The opera Akhnaten
is another impressive work that sounds fairy different.
Also of interest is the album Aguas da Amazonia
. Here, a Brazillian ensemble of self-invented instruments plays the music of Glass. Very "ethnic", very beautiful, and great bubbly fun.