This book provides a thorough description of the evolution of musical instruments from a multidisciplinary point of view from prehistoric times to mid-20th century. It comprises most historical epochs and geographical regions.
Good things about the book: it's a classic, very comprehensive (except for the electrophones), very clear in the technical descriptions, nice figures and pictures (although not enough for the many instruments discussed), it's a must-read for people interested in musical instruments and its evolution. I've learned a lot reading the book.
Bad things: it's aged in many respects. The language isn't politically correct when referring to some countries and peoples; it's too European-focused; gives too much importance to several simphonic orchestra instruments in comparison to other very popular instruments (the guitar family and its evolution is poorly treated, for instance); the electrophones are very badly treated and with some contempt. I know this is a 1940/1968 book and the electromechanical and electronic instruments were still in it's infancy, but some were already there (theremin, ondes Martenot, electric guitar, early synths) which would have deserved a much better treatment. Finally, I didn't like the author's remarks about some other people's work. Although he is an acknowledged scholar, sometimes he sounds very petulant.
I also disagree with the author in some technical aspects. For instance, his classification of musical instruments, although universally accepted, has several weak points in my opinion. Many scholars have followed Sachs' terminology too literally; that's why nowdays some people call 'lute' to any stringed instrument with a handle and neglect its original local name. The results is that a Spanish 'bandurria' is a 'lute' as well as a German 'Laute', or a North African 'gnbri'. That is very confussing and wrong.