We've built a small secret room in our house for our children to discover. One of the features in the room is a bookshelf where we're putting secret room themed books.
We've started to make a list of books, but haven't gotten too far beyond the obvious -- Alice in Wonderland, The Lion, the Witch and the Wardrobe, The Diamond in the Window. I'm fine with books that are not necessarily about secret rooms, but have some secret area element (The Mixed-Up Files of Mrs. Basil E. Frankweiler or The Secret Garden or the Harry Potter books, for example). We're primarily looking for children's or YA literature. Suggestions?