I remember reading a story of a boy finding a settlement of various creatures on a tree. Some points which may help in figuring out the book are as follows.:
- The book was in English.
- The title may contain "tree" or "willow".
- The story was at least 300 pages long and was directed for a young audience but was also a perfect adult read.
- I read it about ten years ago.
- I suppose that the settlement always experienced night.
- I think the settlement was lighted with lanterns.
- As the boy climbed the tree, he made new friends and finally reached the clouds.
I can be a little off in the details but would be grateful if someone could provide the nearest match to this story. One may clear any further doubts in the comments.