There’s a fantastic book called Noodle Pillows: A Journey through Vietnamese Food and Culture, by Peta Mathias (Sydney, 2003) which describes Pho a bit.
Great read, and the author is very funny & clever in her descriptions of life in Vietnam.
I learned there that “Pho” is a Vietnamese mispronunciation of the French word “Feu” (fire). It’s great how such a simple dish can be so tasty. it’s delicious.