If you haven't read Pratchett yet I recommend starting with the Wee Free Men, it's the first in this series. The Wintersmith is the first one that I read and is definitely my favorite. You can read them out of order, without too much confusion, but I prefer to do it that way.
In the third of these discworld novels, Tiffany has to determine who her true love is. Being Tiffany Aching and surrounded by Nac Mac Feegles, this simple plot turns into a big adventure.
What I really like about this book is that the main hero this time is not Tiffany. While she seemingly makes a bunch of errors, it is up to the boy she rescued from Wee Free Men, to rescue her.
This book has a lot more mythology than the past and the standard Terry Pratchard hummor. Overall, a great read.
Wintersmith is the lastest in the Tiffany Aching series of stories set on the Discworld, supposedly meant for children. However in this book, I distinctly got the impression that Terry Pratchett had tried to appeal to his older teenager and adult audience. Certainly, as Tiffany has reached 13, the subject matter has become more grown up that it was in the first or second book.
That aside, Wintersmith is a very good book and definitely worth a look if you've read the first two in the series. Tiffany has managed to make the Wintersmith (an elemental- immortal, but not a God) fall in love with her by daring to join the dance between The Summer Lady and The Wintersmith to signify the changing seasons. The Wintersmith declares his love with snowflakes, avalanches, ice roses and icebergs.
Unfortunately Tiffany has to continue all her chores, put up with Miss Treason who is weirder by far than Miss Level who was strange herself, look after Horace the Cheese and accept help from the Wee Free Men, even if she doesn't really want it. Not to mention that she has a crush on the Baron's son, Roland (who likes her too). She really is in trouble now.. it's down to her to do all this and ensure that the Disc gets to have another springtime.
The series is getting even darker still, and even better than when it started. Like A Hat Full of Sky, it's not as funny as The Wee Free Men, but it can still make you laugh out loud. It's hard to see what Terry Pratchett could do to improve upon the first three books in this series with the fourth and final one that will hopefully be out soon!