Terry Pratchett's Bromeliad trilogy is a mix of childlike fantasy and offbeat SF. While the opening book, "Truckers" lags in places and takes quite some time to really get moving, it's imaginative and very funny. Certainly it's a good place to start off with Pratchett's fiction.
Masklin and the other nomes are tiny people who scavenge on the streets, and now there are only a handful of them left. In an act of desperation, they climb into a lorry and ride to... The Store. Also known as Arnold Bros (est. 1905), where a complex civilization of nomes (about two thousand) live in semi-peace and prosperity. They either are dazzled by the idea of "Outside," or insist that the whole world is in Arnold Bros (est. 1905).
Seemingly, everything is fine for Masklin and his friends, especially when the mysterious Thing (a black box that is a spaceship's flight computer) comes to life and tells them more about their history. But suddenly their world is disrupted by the news of "All Things Must Go -- Final Sales." Now the nomes must escape the
Tiny people living in a department store? Ones from another planet? That is something that could have bombed so easily. But it doesn't, at least not in "Truckers." Clever plot elements like the sign-based religion (they take "everything under one roof" seriously!) and the department-based clans (Stationari? Corsetri?) keep this unlikely plot afloat.
While "Truckers" is a self-contained story in itself, it has plenty of loose threads (mostly involving the Thing and the origins of the nomes) at the end, for the second and third books of the trilogy. The writing has Pratchett's usual sparseness and wit; the only problem is that it takes forever for the nomes to do anything. At least it's a fun slow ride. The drive near the end is one of the best parts of the book.
Masklin and his nome band (especially the indefatigable, vaguely frightening Granny) serve as a good window into the nome civilization, since they're learning about it too. The better-off nomes are a bit snottier but eager to explore the Outside. But the Thing steals the show; despite being just a computer, it has a better idea than the nomes what is going on.
"Truckers" will delight fans of Pratchett, but you don't need to be a fan already to enjoy this story. While the plot takes awhile to go anywhere, the quirky characters and wonderful worldbuilding make it worthwhile.