The Roald Dahl book about Billy entering a forest inhabited by small people is The Minpins. It was published posthumously in 1991, shortly after Dahl’s death.
Billy is warned not to enter the forest, where a dangerous creature called the Bloodbottler lives. Inside, he discovers the Minpins, tiny people who live in the trees and use miniature flying swans to travel.
The Minpins is sometimes confused with The Magic Finger because both are short illustrated stories involving children and fantastical danger. They are separate books with different protagonists and settings. The Magic Finger centres on a girl with a mysterious power, while The Minpins centres on Billy and the forest.
The book was the last children’s story Dahl completed for publication. Its themes include courage, curiosity and cooperation, while the forest setting gives it a fairy-tale atmosphere distinct from Dahl’s school and family stories.