Can anyone help me with the following:
He was a large man with a patrician nose, high cheekbones, intense blue eyes, and a grim set to his mouth.
Does "grim set to his mouth" refer to the expression of his mouth or what?
Yes. It means that his mouth was neither smiling nor outright frowning; his lips were pursed in somewhat of a straight line, indicating that he is not outwardly happy and he is thoughtful and perhaps always expecting a bad outcome of a particular situation. If you met someone with a constant grim set to his mouth you would probably be intimidated - he would seem humorless and extremely serious.