Pratchett does many things well. One thing he does very well indeed is witchcraft.
Yes, yes there is real magic on discworld, but the witches use something far more powerful to help care for and guide their flocks. They understand how people think, and the people of discworld are just like the people of roundworld (earth).
They understand that people would rather believe a good story than a boring one, that giving the impression of something is often stronger than being something and other strange things.
Headology is a big subject and even in this tale set for children (which is also great for bigger kids - like me) he gently walks us through some of the common human foibles when it comes to believing in things without taking away from the plot or characters.
A classic Pratchett plot of "anthropomorphism causes chaos" is told swiftly and surely by a master of his craft.
Another hit from Pterry. 4 out of 5 stars.