Book Nook in a fireplace that is no longer usable. Very creative use of space. Just make sure to clean that fireplace well before putting any books in there and to seal things to prevent water damage or pests from coming down the chimney.
So i decided I'm going to have have one room in my house dedicated to awesomely weird stuff, antiques, books, etc. Think Hogwart's Room of requirement / A. McQueen's cabinet of curiosities kinda thing.