You want 'Hydrogen Jukebox', 'Einstein on the Beach' and 'Music in twelve parts'. After that.. um.. nah really. It all gets same-y. A much better investment of your ears can be found with Steve Reich (apart from the rather wet 'City Life'), Gavin Bryars, and Arvo Part.

Really though I reckon that it's much more rewarding to follow the solo classical musicians and ensembles than it is composers these days. Vanessa Rossetto, Amy X Neuberg (who collaborates with some amazing groups), Lev Zhurbin, Balanescu Quartet, Bartois, Aki Takahashi etc.

But wtf, ignore me, cos tbh I've given up contemporary classical in favour of the blurred territory between it, free jazz and experimental electronic stuff. Its like late 60s modernism is making a comeback.

Glass often gets beaten with the stick that should really be aimed at Nyman. Glass has a lot more going on musicially and has developed in far more directions than most of his minimalist / systems music peers. Well worth having a further dig down.

It's a bit expensive to buy but if you can get "Music In Twleve Parts" from the library that is his large scale 70s masterpiece. More than three hours long. Never dull. The 1996 Nonesuch edition is the one to find.

"Glassworks" is nice too. It's his most rock album. Saw his ensemble play music from this at Sadlers Wells in the early 80s and was totally blown away.

There is also another really nice comp from the late 90s called "Glass Jukebox" which is also on Nonesuch and totally different from "Essential" which is drawn from his Sony records. You can probably pick that up for a song these days.