What’s Up With the Title?
Well, this title is pretty self-explanatory. Let's put it this way: If you totally forgot your homework assignment to read this book and then the teacher called on you to ask what The Witches was about, you could just say "witches," and you wouldn't be wrong. You'd probably be in trouble, but not wrong. (You wouldn't be this lucky with a book like When You Reach Me or Twilight, for example.)
This is a book about witches. What they're like, how to spot them, and how to defeat them. Wait a second, though, what if the book were called Boy? (Side note: Roald Dahl actually did write a book called Boy – it's an autobiography – and in it there were sweet shops and mice.) If the book were called Boy, maybe we'd pay more attention to the boy (or if it were called Grandmamma, to the grandmother). By calling the book The Witches, Roald Dahl is certainly letting us know what he wants us to focus our attention on.