Reddit reviews Music as Discourse: Semiotic Adventures in Romantic Music (Oxford Studies in Music Theory)

The "Dies Irae" refers to a chant used as part of the Requiem. You can hear it here. It's become emblematic in western culture as a signifier of death. Such as in the opening sequence of The Shining. Other user's are talking about pieces that "quote" the dies Irae.

How well-versed in theory are you? Do you understand chord progressions, four part Harmony, figured bass, etc? If not, I would recommend beginning with our sidebar suggestions.

If you have. Might I recommend these areas of further study: Schema theory, topic theory, and semiotics.

• Schema theory looks at music as a collection of "Schemas" that you string together. Schemas are basically soprano/bass progressions that are appropriate to perform various musical actions. It's likened to knowing a list of moves that, say, a figure skater is performing in front of you. While mostly enumerating what these gestures are, how to spot them in a piece of music, there's often a lot of attention paid to what Schemas do, where you would find them, what you expect to come next, and why a composer might choose one vs. another. A good place to get started here is Robert Gjerdingen's Music in the Galant Style.
  
  
• Topic Theory. This is sort of like Schema theory, but it's zooming out a bit. This tries to categorize what kind of music is suitable for what kinds of expression. So, for instance, a Shephard in an opera might be associated with flute music, a low bass pedal point and simple harmonies, and light, dance-like rhythms. This bundle of features would fall under a topic known as the "Pastoral" topic. A book by Leonard Ratner that is sadly out of print is the classic text in this regard, but I might point you to a book such as Raymond Moselle's The Musical Topic: Hunt, Military, and Pastoral.
  
  
• Finally, we have Semiotics. This field tries to codify sets of Musical symbols or signs that are used to trigger certain kind of reactions or convey some kind of meaning to an audience. Kofi Agawu's work is great in this regard. He has works on both classical and romantic music, but since the other two books I've mentioned are all eighteenth-century specific, I'll put his romantic book here: Music as Discourse: Semiotic Adventures in Romantic Music.

That's a huge field. Neo-Riemannian theory is certainly applicable to some of it (and I would point you to Richard Cohn's work), but that's hardly all there is to Romantic harmony.

The role of form in this music can't be understated. These books are the core of modern Formenlehre:

Caplin - Classical Form

Hepokoski/Darcy - Elements of Sonata Theory

Schmalfeldt - In the Process of Becoming

Caplin/Hepokoski/Webster - Musical Forms, Form and Formenlehre

Check out Caplin's response to a Schmalfeldt analysis here.

Kofi Agawu - Music as Discourse provides some semiotic perspectives on the analysis of Romantic music.

In terms of older sources, Ernst Kurth wrote on "the crisis of Romantic harmony" in the music of Wagner but little of his work has been translated from the original German. Here's a collection of Kurth's selected writings in English. Arnold Schoenberg has his own take in Structural Functions of Harmony and Style and Idea.

Wagner's Tristan und Isolde is a touchstone for the study of late 19th century harmony. In addition to the neo-Riemannian literature, Robert Bailey has an essay on the double-tonic complex that you should read. Benjamin Boretz's analysis is also influential. In terms of Neo-Riemannian theory, Laura Felicity Mason's Master's thesis is a good introduction to the most basic operations, and Mohanty helps cut through to the good stuff (even if that's not the point of the article).