More modern music
I sincerely doubt that Terry cares anymore, but I'll continue to give my readership - such as it is - helpful suggestions for modern music. Even though the Bartók predates Das Buch mit sieben Siegeln, though they come from different planets - stylistically.
György Ligeti. Musica ricercata (Aimard, Sony 1996)
Luciano Berio. Sinfonia (Boulez, Erato 1986)
Hans Werner Henze. Das Floß der Medusa (Henze, DGG 1968)
Pierre Boulez. Rituel in memoriam Bruno Maderna (Boulez, Sony 1976)
Dimitri Shostakovich. Cello concerto no. 2 (M. Shostakovich, Philips 1985)
Béla Bartók. Piano concerto no. 1 (Boulez, EMI 1970)
Bruno Maderna. Quadrivium (Sinopoli, DGG 1979)
I make no secret of the fact that I really like Rituel, and I've mentioned it before. Given that phase of Boulez' career, and his seeming propensity to revise and to withdraw, it is astonishing that he "finished" it then and has left it finished. He seems to have one major issue of the piece (the Sony set, coupled with Eclat/Multiples - a work then-unfinished, by the way), and that might be the only Boulez-led version. It is relatively accessible, and a good place to start with Boulez. It might be the canonical "middle period" Boulez piece. You should give a listen to Maderna's Quadrivium, ably conducted by his student, Giuseppe Sinopoli. It provides another perspective on Darmstadt.
Berio's piece is worth a listen, too. It might be the more important, more interesting, piece on here - though it is difficult to overestimate Bartók. It is probably "the" example of real postmodern music. It has a real sense of play, and is deeply allusive. The quote of Boulez' "Don," for example, is beautifully and wittily handled. It deals with Mahler, especially in the third movement, in a very sort of postmodern way. Berio's own notes, dedicating (mon Dieu!) the piece to that paragon of the avant-garde, Leonard Bernstein, are illuminating to a fairly high degree, though you'll probably want to do some research of your own. This is an important and (unsurprisingly) "important" - yes, there is a difference - work, conducted in what has become the reference recording by Pierre Boulez.
The other stuff on my list is, to my mind, a fairly good second helping of 20th century music. More Ligeti, showing his somewhat more ominous side. Shostakovich, of whom I'm still not sure what to think. Bartók, which reminds me again that I need to get some of my thoughts down about him before I get busy. Henze. There should be a post on Das Floß, but don't hold me to it. All I'll say is this: Das Floß needed more than a raft itself. In any event, these are a few more suggestions. The great and terrible thing about 20th century music is that there are so many directions that no one set of recordings will suffice.