Nikolaus Harnoncourt has been one of the most important and influential leaders of the 20th-century revival of Baroque instruments and period performance practice--not just because he's a hardworking and talented musician, but because he's articulate. (He has had to be articulate: in the 1960s and 1970s, most of the classical-music establishment was contemptuous of, if not downright hostile to, his ideas.) The Musical Dialogue, along with Harnoncourt's Baroque Music Today: Music as Speech (to which this volume was issued as a companion), is a valuable collection of lectures and essays laying out those ideas. In the previous book, Harnoncourt discussed his views on phrasing (using short phrases based on the idea of rhetoric rather than on a long, unbroken legato line) and the use of period instruments (not simply because the composer used them, but because, when played well, they reveal more of the music to our ears now).
In The Musical Dialogue, Harnoncourt gets specific: he discusses how Monteverdi, Bach, and Mozart used particular instruments and forms and talks about his own experience analyzing and performing particular works. Among the topics he treats are the various possibilities for instrumentation in Monteverdi's works (and why there are so many), how he figured out what exactly was the oboe da caccia ("hunting oboe") called for in some of Bach's vocal works, and the performance history of the St. Matthew Passion and the Mass in B Minor and how that history has affected audience reactions to modern-day period-instrument performances of the works.
Perhaps most valuable are Harnoncourt's discussions of Mozart: for example, how the meaning of tempo markings such as "allegro" and "andante" has changed from the 18th century to today, as well as the many and varied conventions of phrasing that were widely understood--and therefore were not written out in Mozart's scores and performing parts. One caveat: these essays were written in the 1970s and 1980s, and there are a few cases where subsequent events have overtaken Harnoncourt's observations. For example, Monteverdi's large-scale works were probably not orchestrated quite as freely as he indicates; there are now a number of mixed adult choirs that can reproduce fairly well the pure tone of boys' choirs (which was not true in the 1960s and 1970s); and present-day composers and audiences are no longer completely alienated from each other, especially in the United States. Nevertheless, the observations of this important, influential, and idiosyncratic maestro make fascinating reading. --Matthew Westphal