We're not talking about the kind of awe and amazement that comes from looking at something really beautiful. In "There's a certain Slant of light," the speaker's amazement comes from the mysteriousness of that slant of light. In fact, even the landscape is amazed by the light and "listens" upon its approach while the shadows "hold their breath." Pretty…uh, amazing, eh?
What's particularly amazing about that slant of light is the fact that it's impossible to define in any absolute sort of way. It's amazingly hard to pin down, which makes it somehow even more amazing.
All of those dashes and odd syntax reflect the speaker's difficulty in defining the awe she feels because of that slant of light. Awe-some, eh?