The problem with this huge book is that you could only really understand much of it if you already knew what it was trying to say. It is loaded with jargon; perhaps this is necessary to keep the treatment short, but too often the author writes as if the thing he is supposed to be explaining had already been explained. Not only that, it is full of foreign language titles and quotations that are left untranslated, and even when he uses English the author often manages to be obscure. The material is present by topic, rather than by composer, and in very short snippets so that the treatment feels bitty. The comments about any particular composer could be split between a dozen different sections; it feels as though the overall importance of Bach, say, or Mozart, slips through the cracks between these bits. After all it is primarily as a composer, not as individual pieces of music, that a composer has an impact - both in his own time and to us today.
When it says 'music' it means, predictably, Western classical music, although there are brief but respectful nods to the music of other regions. There's nothing about any kid of pop/rock, and folk music is mentioned briefly only where it affects the development of classical. I saw a review which said it gives short shrift to modern music; I don't think it does, it just doesn't treat the modern era at greater length than others as is often the case. It does come to an end around the middle of the C20th; the author, writing in 1979, says he didn't feel able to identify the most significant music of his own day. And actually, there isn't that much of interest to show from the last 40-50 years.
For a book like this to be produced by a single author is a mammoth achievement of learning - understanding classical music is the work of a lifetime, I don't know how anyone ever gets round to actually creating it - but there aren't enough concessions to the supposed target audience, the legendary 'intelligent layman' (or woman). Probably it would be useful as a reference work for undergraduate music students, who will presumably be familiar with the terminology; and if you wanted a book to point you towards the key works of the repertoire, you could do a lot worse.