1999 P & D Quarters?

Because they have been out the longest !

With out knowing exactly what it says in the book, it would be hard to tell why
they say that.

But .. many of the early State Quarters are hard to find in better/higher grades.
This has been gone over many times on this forum.

Bottom line >> in my opinion >> is
Collecting quarters from 1999 and up, while is fun to put sets together.
To have sets that are going to increase in value, you neeed to have
coins with the highest grade you can find.

As Eaglefoot says circulated State Quarters will only be worth 25 cents.
Of course there may be one or two exception like error coins and the
WI high and low leaf.

A example of what I mean is take a look at a price guide, like numismedia.
It will show that Georgia quarters in MS67 or better have listed value of
over $100. There are several that fall into this category, because they are
hard to find in higher grades.

In the future there will be many collectors wanting to put together a set
of quarters, the ones they will pay the most for will be high grade UNC
coins.