One of the reasons I like Jonathan Kramer's The Time of Music so much is that it's one of the only significant music theory books that accommodates drone music. Kramer devotes one of his final chapters to "vertical music," which he characterizes as timeless music, without linear causality, where each event exists only for itself. Kramer's field of expertise is contemporary classical music, so his examples include early music of Philip Glass, various works by John Cage such as the Variations and Atlas Eclipticalis, some music by Frederic Rzewski, much of the work of La Monte Young and Morton Feldman, and so on. Most of the pieces he discusses are not really drones, but it is no stretch consider them a subset of vertical music.
Earlier in the book, he has a chapter on how technology has changed our view of musical time. This is probably the most dated chapter of the book (which was first published twenty years ago). While Kramer highlights the composer's ability to create more complex works (especially in terms of rhythm) and to achieve an immediacy between a piece's conception and hearing the results, but overall he disparages the computer's ability to create interesting music except in the direction towards increased rhythmic complexity.
Paradoxically, it strikes me how much vertical music seems to require advanced technology. Or, to put it another way, how little drone music uses only acoustic instruments. La Monte Young used brass instruments for The Second Dream of The High-Tension Line Stepdown Transformer. Cage's last compositions, generally called the Number Pieces because the titles are the number of performers, are especially remarkable for the way that, when the instruments have appropriate sustaining capabilities, they create beautiful drones. Saxophonist Ulrich Krieger provides superb examples with John Cage's Four5, scored for a saxophone quartet. Krieger has recorded the piece twice. Cage permitted multiples of the sax quartet, so on one of the recordings Krieger overdubbed himself five times.
Nevertheless, Cage and Young are exceptions. Most contemporary drone music require computers, or at least a good electronic music studio (for the years before computers became ubiquitous). With Kramer's focus on composed instrumental music, he didn't consider the investigations of the micro time scales that a diverse set of musicians have used since the 1950s, as documented in the early chapters of Curtis Roads' definitive book on the subject, Microsound. Other musicians have pursued this and similar directions, all of which contribute to Kramer's timeless music. Michael Northam combines music into hundreds of layers to produce his works of suspended time, an act difficult to contemplate without the aid of a computer. The Hafler Trio's epics sourced from recordings from a single person's voice, ditto. The list could go on, even including more mainstream ambient musicians such as Robert Rich and Steve Roach. While this doesn't negate any of Kramer's work, it shows the opportunity for extending his work into myriad artistic endeavors.