Here is one balanced appreciation. The Wikipedia page is patchy but offers lots. If you’re only going to buy one or two, I say start with Mantra and then move on to Stimmung. The hard cores should seek out Spiral. Gesang der Jünglinge is perhaps the most influential and seminal work, or perhaps Hymnen, order them here. The junk includes the Helicopter Quartet, Tierkreis, anything with an American Indian theme, and most of material from the operas. Gruppen you probably had to hear live. Momente has compelling parts but it makes me giggle. The piano music was good, although for me never a highlight. It’s easy to take potshots at his pretentiousness and stupid politics, but he created more memorable and distinct sound worlds than any other twentieth century composer, except possibly John Cage. It’s hard to imagine music without him. Miles Davis and the Beatles would agree, and they were pretty smart guys.