Ahoy there matey
The answer would be between A and B, but I'd choose B because A doesn't exactly influence his beliefs if you know what I mean.
ARRRRR!
So are you asking us to actually pick a book for you? Cause if so The Secret Life of Walter Mitty and The Story of an Hour is good!
Thoreau mostly uses imagery to illustrate time in the excerpt. The second sentence is saying that he is in the midst of time/living, and he can "detect how shallow it is," meaning that he is aware that life is short and that the end is inevitable. Thoreau also describes time as being fleeting, but ever-present.
I hope this helps!