The Minpins by Roald Dahl.
 
The Minpins by Roald Dahl is a classic children's story about a young boy named Billy who ventures into a magical forest and discovers a hidden world of tiny people called Minpins. Billy befriends the Minpins and helps them defeat a fearsome monster, the Gruncher.
 
Dahl's final work written for children was posthumously published the year after his death.