The confusion may be because you speak about different “pronunciation”. To most people that means that you use different sounds, or accent different syllables. But these things do not change, no matter how you are using the word. What DOES change is how much you EMPHASIZE the word in a sentence.

1) “Pretty” can be used as an adjective, meaning “attractive, beautiful”, as when someone says “She’s a pretty girl.”

It can ALSO be used as an adverb, in combination with an adjective. In this case it means “fairly, moderately, to a reasonable degree”. It does NOT mean the same thing as “very”.

And example would be “I’m pretty hungry” meaning “I am fairly hungry.”

(This is not the same as saying “I am pretty and hungry”. In this sentence “pretty” and “hungry” would both be adjectives.)

2) But the example you give is of the adverb, not the adjective use. And the adverb and adjective are pronounced exactly the same way.

So perhaps the book is referring to how putting a special EMPHASIS on the adverb when you speak can change the meaning of the sentence slightly.

Normally when you use “pretty” as an adverb, the main emphasis is on the adjective that follows. So, ifsomeone says “It was a pretty good day” they are being positive. The idea is that they are satisfied with how good it was, perhaps even surprised at how it was better than they expected.

But sometimes a person will strongly emphasize the word “pretty”, saying “Well, it was PRETTY good day.” In that case, the person is emphasizing that it was only “fairly good” INSTEAD of “very good”. They are suggesting that the day could have been better, and maybe they are complaining, suggesting that they think it SHOULD have been better.

So, this EMPHASIZED use of “pretty” makes what you are saying with the adjective WEAKER not stronger.

Very was used and it seemed extreme, so pretty replaced it, as so many words do, to be less worrisome. A very long trek would a pretty long trek. Must not offend delicate sensibilities.

We cannot possibly say true what we mean in this day and age. No, we must make things seem safe and happy. Ergo, bastardisation of words to make people feel safe and happy.

its uses are two

she is a pretty girl

we are pretty late