The Rest Is Noise
I was much affected by this book when I first read it. It is an immense book and draws upon extensive research. Present-day composers struggle to find their style. At the beginning of the 20th century musical composition was confident and much respected by large sections of the population. By the end of the century a mass of styles prevail with not style one standing out as to the way forwards. In effect it's "Do what you want. It's all noise."
If you want to know how in the 21st century we got to where we are then this book is an absolute must.