I believe that a Young Adult category would be well worth considering. A substantial number of such works are equally enjoyable and challenging for adults. The Dark Angel trilogy by Pierce is an example; the original first three Earthsea volumes--especially the second book, The Tombs of Atuan--which can be read as a stand-alone--is another.
The Heart's a Heavy Burden--Howl's Moving Castle
"Time and Relative Dimension in Space"