Nuance deals with the atmousphere a word or expressions gives off.
Traditionally in business etiquette, the phrase "Good Question" affirms to the asker of the question that their question is valid and worth asking.
The book your reading seems to explore the idea in more depth, suggesting that you can seem unsure by saying "Good Question".
Of course, this is quite true, as any unnecessary pause or "time-creator" such as saying "Well..." or "Thats Interesting" makes you seem a little unsure of the answer you are going to give.
Its also worth noting that saying "Good Question" in most ways makes you sound unsure about the answer, to be sure that you don't put across any insecurity make sure you say it with confidence, and maybe a smile. Alternatively, you could just not say it, and go straight into the answer, which is both concise and impressive.
Hope that was of help.