This is the kind of book that Fantasy fans who don't read YA will claim is YA, because the protagonist is the right age and it doesn't stretch their comfort zones. It is not YA. It is Fantasy. This isn't a bad thing by any means, but maybe don't recommend it to someone who is looking for a good YA read as it will just make them suspicious of your future recommendations.
YA tends to have fewer giant dick jokes in the first chapter, for starters.
And now that I've gotten those two points out, let's talk about what a great book this is. This is the 4th Tiffany Aching book, and the i-have-no-clueth Discworld book. All of them are good.
I Shall Wear Midnight starts strong (well, once Pratchett gets enough dick jokes out of his system in the first chapter), showing Tiffany working to fulfill the role she's trained for. Wearing a pointy hat may be a visual metaphor for being a woman with a career, and this work, like other Witches novels in Discworld, continues to examine issues women still face by daring to have opinions and authority and jobs. The approach to the casual sexism of society makes it as much a spiritual successor to Equal Rites as it is a plot continuity sequel to Wintersmith. Unlike Equal Rites, Tiffany is a Witch, clearly a woman's role, but what kind of woman, exactly? That's the question on the tip of the tongue of everyone she interacts with.
Sadly, while it does show some religious based discrimination, compared to other Discworld novels, it feels like it's pulling it's punches in this regard. And I found the antagonist rather unsatisfactory on a metaphorical sense. Like the bad behavior of too many people gets easily swept under the carpet by literalizing mob behavior as a distinct entity.
But, whatever, I'm still waiting for a Netflix Original treatment of this series. How great would that be?