A Hat Full of Sky is the sequel to the Wee Free Men. It is about an eleven-year old girl named Tiffany Aching, who is training to be a witch, and the Nac-Mac-Feegle (Wee Free Men), who are fairies (but do NOT call them that unless you want to be seriously injured).
Tiffany is a very unusual witch, because she's from the Chalk Land. In fact, Tiffany is actually the ONLY official witch of the Chalk. She is also the Hag of the Chalk Land, which means that it is her job to protect the Chalk. (She tells it what it is; it tells her what she is.)
When something evil comes to the Chalk, Tiffany has to make it go away.
-- Fizzy, age 11
An adult's view...
The Hat Full of Sky of the title is Tiffany's witch's hat. For services rendered in the first book in this series (The Wee Free Men), Tiffany is anointed with the invisible but meaningful hat of the Head Witch. And throughout the book, Tiffany wrestles with whether to proclaim to the world that she is indeed a witch (by putting on a hat that everyone can actually see), with whether to accept the responsibilities as well as the honors accorded to witches, and with whether she is qualified to succeed her illustrious grandmother and talented enough to conquer the evil and the self-doubt that threaten her land and herself.
Pratchett seems to think that, yes, Tiffany is indeed talented enough, and she needs to know that, but no need to admit to the rest of the world that she really is magical. So, here's a child allowed to be special enough to save her Land, but she's not to be too special. And Tiffany is not the only one who is pressured to conform. Her Teacher is another special one. As the plot progresses, the feature that makes her special is mitigated so -- no more problem -- the specialness is gone.
My daughter really enjoyed reading this series, as did I. But as the end of this book approached, I came to realize that if one wears an invisible witch's hat, one does not have to admit to oneself or to anyone else that one is special/magical. Kind of like the Rainbow Fish, who gives away all his colorfulness, so that he can be just like all his new friends, who were not very accepting of his uniqueness. Is this the message we want to convey to the heroes in our society? (It's fine to sacrifice your all for our good, but please don't admit that you have qualities that we do not have.) Just asking.
So, anyway, both my daughter and I also enjoyed reading Stargirl, which weighed much less on my mind than Hat Full.