Books featuring scenes INSIDE the tree (not tree houses that sit in/on a tree). No, we're talking INTERNALLY. For example:

- there's a door in the trunk of the tree that you can enter (doesn't necessarily need to lead into the heartwood of the tree, but could be a portal, for example)
- there's a hidden (miniature?) fort/city/dwelling beneath the tree bark
- there's a knothole with something hidden inside the bole of the tree (for example, in To Kill a Mockingbird - there's a hiding hole for treasures)
- the tree obscures a tunnel entrance or cavern beneath it (tree acts as entryway of some sort)
- a hollow tree you can crawl into
- a tree you can pass through, a la in Harry Potter and the entrance in the pillar to platform 9 3/4
- something is hidden or located under the roots of a tree
- cover depicts a hollow tree or a door in the tree, but doesn't necessarily reflect the story

You know that scene in The Princess Bride where they go down to the dungeons of Evil Prince Humperdinck to find Westley "mostly dead"? And the secret entrance to the stairwell is inside a particular tree? That sort of thing, too (I added the book, although now I'm worried that the tree entrance was for the movie version, only. Because the movie didn't have the killer invisible tiny spider on the door handle scene...).

RELATED LISTS:
Magic Gardens: https://www.goodreads.com/list/show/1...
Magical Trees (generally): https://www.goodreads.com/list/show/1...
Fiction - Portal is in a Forest: https://www.goodreads.com/list/show/1...