Reddit mentions of Music as Discourse: Semiotic Adventures In Romantic Music (Oxford Studies In Music Theory)

Depends on what sort of rep you're in to (into?). I read a lot of popular music scholarship. These are books that I have either read or am going to read, in no particular order:

Brad Osborn - Everything in its Right Place: Analyzing Radiohead

Robert Walser - Running with the Devil:Power, Gender, and Madness in Heavy Metal Music

Walter Everett - The Foundations of Rock

Walter Everett - The Beatles as Musicians: The Quarry Men through Rubber Soul, Revolver through the Anthology

† Understanding Rock: Essays in Musical Analysis, ed. John Covach and Graeme Boone

Richard Middleton - Studying Popular Music

Allan Moore - Song Means:Analysing and Interpreting Recorded Popular Song

†Analyzing Popular Music, ed. Allan Moore

† = A bunch of essays crammed into a book. These are nice because you can read a 20-50 page study on a topic and move on.

I'll recommend these as more general reading, geared toward classical music but useful in other styles as well:

William Caplin - Classical Form: A Theory of Formal Functions for the Instrumental Music of Haydn, Mozart, and Beethoven (Every researcher in the field knows or should know this one. I would also recommend Hepokoski and Darcy - Elements of Sonata Theory, which is the other big form book, but it is a veritable tome and hardly light reading. Not that much of what I've recommended is very light.)

Janet Schmalfeldt - In the Process of Becoming:Analytic and Philosophical Perspectives on Form in Early Nineteenth-Century Music (I haven't read this yet, but it's in the same universe of form studies as the above.)

Kofi Agawu - Music as Discourse: Semiotic Adventures in Romantic Music (Agawu has written a lot on various topics and should be on your radar.)

Alfred Mann - The Study of Fugue (Broken up into two parts: part 1 is a history of fugue, part 2 is about fugal technique.)

I'll point you toward this one since it is totally awesome and helped me to understand medieval liturgical music theory and culture (though you might find yourself lost if you are not already familiar with some of the concepts):

Anna Maria Busse Berger - Medieval Music and the Art of Memory

You might also consider ordering a hard copy of some journal issues, as these can be quite compact and give you a lot of breadth.

Journal of Music Theory (JMT)

Music Theory Spectrum (MTS)