So many great quotes already, and, yes, most of them from the conversation between Tiffany and Miss Tick.
My daughter is 26. I just ordered the book to be sent to her from Amazon, because she will love it, and because I can't read it to her at bedtime at this point. 
I love books like this. Loved Going Postal. Loved Making Money. Probably will love this more. 
You are in for such a treat. The Moist Lipwig books are great but are just the tip of the iceberg with Pterry.
What are your thoughts on headology and how well drawn the female characters are, J-Mack?
Well, headology is a nice scheme for world-building and having access to any storyline Terry might have wanted. I find the distinction very interesting between Fantasy that depends on our RW "western" stories (PTerry, Piers Anthony [not a name often mentioned here], Goldman, Tolkien, J. Williams) and those that want to carve out new territory (Mirror Empire, Malazan, Sanderson to name a few). It's personal taste, but I find the "traditional" richer if really well done. But that doesn't mean I don't find the explorers fascinating.
But the "traditional" taps into our archetypes most directly. I've never read Bruno Bettleheim's
The Uses of Enchantment, though I'm interested by what I understand the premise to be. It sat on my TBR for a very long time, until a bookshelf thinning some years ago. But the idea is that the original Grimm stories and those like them address the terrors and reality of life in the form of story, and that this is important. Contrast this to more sanitized children's movies and literature. (Han Christian Anderson is delightful, but I don't recall him being dangerous.)
So I bring this mindset to headology, for what it's worth. PTerry gets to address real world stuff in humor and headology. Love it.
OK, the ladies. It is
the flaw of my beloved Tolkien that the ladies barely get their due. But at least he respects them. Others on this site have read my increasing annoyance with Sebastian de Castille in
Traitor's Blade over his female characters. Yesterday, went to see
Kingsman, which was a hoot, but did anyone notice how the girl agent, who is our hero's equal, is shunted off to down the satellite, act frightened, and talk mom out of throttling the helpless little girl? What's up with that? She doesn't even call 'bullshit'.
So, on topic. PTerry's ladies in this book so far are great. I haven't read all that many Pratchett books, but I can't think of a poorly rendered woman character. Love Granny especially. Love Miss Tick for being smart and limited, i.e. realistic.
My feelings for Tiffany... love her, absolutely. Frying pan, well there's me, etc. She's also a wonderful version of what I see as a "type" in YA and children's literature: the plucky girl (boy). Lucy in
Lion Witch Wardrobe is her ancestor, though there was Dorothy before her. I think this is often the lesson of YA books: you don't have to be popular or pretty to be the best you can be.
Meanwhile, she's not perfect. She is a little prideful, let's say. There's a perfect line, can't remember fully, and the narrator says: "And that tells you more about Tiffany Aching then she'd prefer that you know." Lovely.
I actually think there's more I could say here, and say better, but am running out of time.
Fortunate for you all.
