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.