http://imslp.info/files/imglnks/usimg/4/4e/IMSLP59385-PMLP121821-Avison_essay_on_musical_expression.pdf

This is a very handy book for those of you that know that something is wrong with modern music but are not well-versed enough in music to make a solid critique of it. Reading this book will help you know how to crush anyone that holds that modern music is not inferior to older styles. The book is old (written in the early 18th century) and the author, Charles Avison, heavily critiqued even the music of his own day as being deficient and base (Avison himself was a very able, but old-fashioned, composer in his day).

A note: The book is in an antiquarian type-face so if you haven't read a book printed with such type before, it may be a little difficult. You get the hang of it after a few chapters though. A lot of the spellings are different, and the "s" is printed as "f".

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Dulcamara, this book is in the public domain and the author is long dead so you don't need to worry about copyright or stealing his money.  

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The book may be a bit difficult to understand for those not familiar at all with music and musical terms, but Avison's writing is very clear and he explains everything very well.