This book is a bold departure from the mainstream view of the progressive complication of harmony in classical music from 1500 to 1910. I have read through several standard theory texts--and gone through theory sequences at two music schools--and the usual explanation is that people's ears mysteriously got used to wilder and wilder dissonances. If any reason for this development is adduced, it is the rise of industrial capitalism and its attendant folkways. While this may satisfy my curiosity at a sociological level, it doesn't shed too much light on things that interest me more at a purely musical level--why, for instance, the bVI key area became such a hallmark of music of the second half of the 19th Century, particularly in Brahms.
Van der Merwe takes the view that a great deal of this practice percolated through from popular and folk music of the day and the Gypsy "Fringe" of Eastern Europe. Vienna, in particular, was open to such influences, being a crossroads of a multi-cultural empire; and many of the leading composers of the day made it their home. He demonstrates convincingly that many of the "strange new harmonies" of the day were simply borrowings from the Fringe--things you could hear in the street if you listened hard enough.
He further shows how a lot of folk music (including children's teasing songs, our 'earliest' music) has a pentatonic structure, and that this, too, was important in art music one would not usually think of as pentatonically-inspired.
Van der Merwe, who is not a professor of music, holds marked, unorthodox views, which are entertainingly expressed. Yet he writes with great erudition and without crankishness.
At times the thread can be hard to follow and one must go back over sections a few times. This isn't a book to plow through! But if you're looking for a new explanation of why music developed as it did, this book will open new vistas for you. (Note: there are many musical examples in the book--if you don't read music, you will be lost.)