Even among conservatives Patrick J. Buchanan is a somewhat controversial figure but his 2002 book The Death of the West is a pretty good account of how our civilisation is being destroyed from within.
Demographic collapse and massive Third World immigration are the most obvious threats but Buchanan also takes aim at the climate of political correctness and the self-hatred of liberal elites.
He provides a good rundown on the long history of cultural marxism which is of course the real key to the decline of our civilisation. Also in his sights are modernist artists with their agenda of misery, ugliness, degradation and hared of the West (an agenda shared by modern Hollywood).
Unusually (and pleasingly) for a book of this type Buchanan includes some tips on how to fight back.