I just finished the newest Discworld book, I Shall Wear Midnight, the fourth and last in the Tiffany Aching subseries. I loved it, it was an amazing finale to the life of Tiffany Aching. Through the series she has gone from age eight to age seventeen, and matured and changes greatly. Pratchett's characters are not static, they all change and grow (not always for the better), and experience things that make them who they are.
While the Tiffany subseries was originally for older children, it has progressed through the age levels and with I Shall Wear Midnight become almost as adult as the rest of the Discworld arcs. I liked that progression and I could see it happen throughout, similar to the Harry Potter series.
The book itself focused on the young witch Tiffany Aching, her relationships with others, and her "steading"- the region that she serves the people, animals, and land of. This time she has to fight off an evil greater than any she has faced before. While that sounds cliche, the actual story is not at all. There are many cameos of characters from other arcs of the Discworld series who don't just appear but add to the story in unique ways. I absolutely loved the mature turn this book took, and love what Pterry did with it.
tl;dr: I Shall Wear Midnight by Terry Pratchett. Amazing. But read the other Tiffany Aching books first.