It seems I simply can't pick up a Discworld book without finishing it within three days or less.

The Tiffany books were my introduction to Discworld, and they, and the witches in general, are really still my favorites. I don't think I've ever experienced that elusive sense of being represented in fiction as strongly as when I read about Tiffany Aching mispronouncing words because she'd only ever saw them written down. More than that, I admire her (and Mistress Weatherwax). She's responsible and practical and decisive, all virtues that, in my opinion, don't get enough attention in fiction. Probably because authors don't naturally tend to be the practical sort.

I'm continually amazed by how well Pratchett writes women, even the clique-y, bizarre internal politics of tween girl friendships ring true. (If you weren't a Petulia or Anagramma, you've met them.)
Warm, insightful, incredibly funny... It's a Pratchett-book, what more do you need?