I think Tiffany Aching is the sub-"series" of books I have read the most, even though she isn't my favourite. Tthat is not to say, though, that I don't like her - on the opposite, I absolutely adore Tiffany and the Feegles! Its just that reading Tiffany's personal story arc takes way less time than reading, Vimes, for example, or Granny Weatherway - okay, bad examples, maybe, since Vimes has almost as many cameos as Death and Granny Weatherwax's books include the TA books.
What I wanted to say, perhaps is: I love Tiffany Aching's books. In fact, the second Tffany Aching book was what got me back into Discworld after 10-15ish years of not having read another one (my previous experiences were, let's say, not crowned with success). I have to admit, the first time I read it I couldn't really appreciate "I Shall Wear Midnight" for reasons you all might guess (Tiffany/Roland *cough*), though I recently reread it, and it *is*, and always was, a great book. Even though I haven't yet read her book, I knew a fair bit about Esk, and so was thrilled when rumors said she'd return. In the end, I thought it was the perfect ending to Tiff's story arc, so I was a bit puzzled when a new book was announced, but I still look forward to it.