A few facts you need to know about her:
1. She goes by Ursula K. Vernon when she writes graphic novels and kids' books, and they're all fantastic and quite well-known (see: Harriet the Hamster Princess). Anything written as T. Kingfisher is probably not appropriate for readers younger than, say, 13-ish? It's not just that there might be sex, and possibly more daggers through eyes than you might want a younger reader experiencing, but her themes are deeper, her focus is more ... philosophical, for want of a better word. The world is a more complex place, both grimmer and more humane.
2. She really gets fairy tales. As in, she has obviously imbibed so many, from so many different traditions, that folk-tales run through her veins, but also she has a genuinely weird imagination that works easily with fairy tale logic. The things she comes up with!
3. She is hilariously funny.
Clockwork Boys and The Wonder Engine—really just one novel broken into two—are a bit different than the others of hers that I've read. Not so much fairy/folk-tale inspired, but more of a re-imagining of the fantasy quest, with a healthy serving of vaguely steampunky Lovecraftian horror thrown in (and if you think that sounds weird, you're right!)
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Then they go off to get the horrifically inventive safety measure that ensures they have to complete the mission (don't want to spoil it for you, but if they don't die trying they will just die painfully), and they meet their team-mate the assassin ("I don't like people unless I'm stabbing them."). Then they meet the 19-year-old self-righteous scholar who's never been outside his monastery and thinks women will turn his bowels to water. Who is also coming on the mission.
So it's the most extreme version of incompatible quest-mates I've ever encountered: the banter and drama is endlessly amusing. (As in, every second sentence made me smile, and every second page I was laughing out loud. Then came the scene with the horses, and I was gasping and wiping tears from my eyes the whole chapter.) Kingfisher knows how to structure a running joke, and that's basically what the whole first book is. (Something along the lines of "I wonder if we're going to kill each other before we even get to the place where we're likely all going to die.")
But, by the end of Clockwork Boys, the trust the four of them have built in each other is as moving as their differences were funny, and I genuinely cared about each one of them and about the fate of the team. Slate, Caliban, Brenner and Learned Edmund kill me in all the best ways, whether they're trying to kill each other or not.
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Also, most hilarious torture scene ever written.
I think if you like Terry Pratchett you'll probably like T. Kingfisher. She's an auto-buy author for me now; I have yet to be disappointed in anything she's written.
The Clocktaur Wars is a balsamic/maple braised pot roast with red peppers and sweet potatoes (and I threw some kale in because you can hide kale in a sauce this rich and flavourful). Sweet, tangy, not exactly your normal pot roast, but still hits the pot-roast receptor in your brain most satisfyingly.