DD 13 read a book when younger and is dying to read it again.
In the story, a rich girl is 'kidnapped' by a female teacher and taken into a forest. A girl goes looking for the rich girl and enters a new world through a gap.
There is no sunlight ever in this world.
This new world is like a forest and there are pig-like creatures that protect her from dangers in the forest.
There is a castle on the edge of the forest which has a cellar. The rich girl is taken there, has a tube attached to her head where her happiness is extracted. The happiness is turned into powder that the villagers use to their benefit.
Possibly the rich girl dies, not sure.
Does this sound familiar?? Thank you!