IT's a fast read, btu it isn't necessarily just for kids. Much like Garth Nix and his Abhorsen books, aq good author can put some deep themes into a juvenille book. Pratchett has sometimes had naive adults use phrases like "Wouldn't it be nice if everyone was nice." Adn while that's a nice dream, all but the most naive know that'll never happen.
One of Pratchett's recurring themes is racism, but wrapped and disguised as speciesism usually Humans against Dwarves or Trolls. This time it Humans against Rats. And while it's a twist on "The Pied Piper of Hamlin", this one is also about getting outside your head and realizing that while life may be a story, it's not always going to follow the narrative.
Recurring themes and characters appearing in this book:
Death of Rats
Purple Wobbly Bit
Sgt. Doppelpunkt (Colon)
Cpl. Knopf (Nobby)Unseen University