Reddit mentions of The Cambridge History of Western Music Theory (The Cambridge History of Music)

Usually a good idea to include:

• The Cambridge History of Western Music Theory
  
  
• Strunk's Source Readings in Music History
  
  For the kids to read:
  
  
• Music in Ancient Greece and Rome
  
  
• On the interpretation of Greek music (free on archive.org)
  
  Accessible primary sources:
  
  
• The Manual of Harmonics of Nicomachus the Pythagorean
  
  
• Ptolemy Harmonics: Translation and Commentary
  
  You might want to get/read:
  
  
• Apollo's Lyre: Greek Music and Music Theory in Antiquity and the Middle Ages
  
  
• The Science of Harmonics in Classical Greece
  
  
• Ancient Greek Music
  
  
• Ancient Greek Music: A New Technical History
  
  
• Greek Reflections on the Nature of Music
  

What you're describing is musicology. Richard Taruskin is a big name author, but reading his work is a commitment. Without knowing your area of interest (11th century church music, 15th century frottola and madrigals, 19th century salon music, etc.), I can't make any concrete recommendations. Know that there's a lot of diverse literature out there though.

There's also history of theory. /u/dcgrey, allow me to introduce you to the blue brick.

Music composition in medieval and early renaissance, and classical period can be analysed as pattern based.

Some good books - https://www.amazon.com/Music-Theory-Problems-Practices-Renaissance/dp/0816669481

https://www.amazon.com/Music-Galant-Style-Robert-Gjerdingen/dp/0195313712

https://www.amazon.com/Cambridge-History-Western-Music-Theory/dp/0521686989

The same can be said for many traditional/ethnic music styles. (Get any good book on X folk/pop/ethnic style depending on your interests,)

>It's perhaps the least romantic gift ever

Hahahahaha! I would definitely agree, though—I think the CHWMT would be an excellent book. If she goes through any sort of History of Theory course (which most PhD programs do), I can't imagine she wouldn't use this book. Even if she didn't have such a course, this book is a collection of (with perhaps one exception) excellent essays written by top scholars on almost every major theoretical approach or issue in the history of Western music.

I won't speak for other sub-disciplines—vornska's suggestions are definitely some of the central books in present theoretical studies—but let me make some suggestions for books more oriented towards Schenkerian analysis.

Schenker's Free Composition — this is Schenker's magnum opus in which he lays out his mature theory. For any Schenkerian, this is definitely a Bible of sorts, and a must-have. Just be sure, if you end up purchasing this, to get both volumes; one volume is the text and the second is the examples. You can also find the hardcover first English edition, sometimes even for less than the price of the two paperbacks.

Cadwallader and Gagné's Analysis of Tonal Music: A Schenkerian Approach — this has become the standard textbook for teaching Schenkerian analysis, and I still find myself referring to it after years of Schenkerian studies. A somewhat dry but very clear and beneficial book.

Schachter's Unfoldings: Essays in Schenkerian Theory and Analysis — Carl Schachter is one of the greatest Schenkerians; nearly everybody who's anybody in the world of Schenkerian analysis studied with him. This book is a wonderful collection of some of his greatest essays. His writing style is exceptional and his analysis are some of the best I've seen.

The discipline you're looking for is a little more specific; you're looking for the History of Music Theory. Luckily for your boyfriend there's this glorious/awful son-of-a-bitch book.

The "how" and "why" of music theory is way more complex than evolution. Spend a little time with this book, and you'll find yourself scoffing at the idea of a teleological view of music. The tl;dr truth of why we do music the way that we do is because theory is a hot mess of style conflicts, nationalism, culture-wars, dogma, religion, mathematics, and science.

This book will be difficult to understand without a solid background in theory, but perhaps getting some of the "whys" might motivate you both toward investigating the "hows" and "whats" of musical construction.

source: PhD. music theory.