Books that play with the "book" as a medium or are otherwise self-reflexive/meta.
My current criteria for this list are (1) does the book highlight its own textuality consistently or (2) do the metafictional elements add significantly to the themes explored or (3) does the book's use of discourse and form (e.g., narrative situation, layout) significantly differ from the norm?
Make sure to comment your suggestions with a small explanation even if you're not 100 % sure if they fit!