What are your "required readings" for a game designers, that are not design books.

For instance, I think any systemic designer benefits from knowing the rules of DnD and MtG, including how they evolved and (most importantly) why.

@tomgrochowiak I found "the humane interface design" and to be very insightful, and the book on impro theater by Keith Johnstone. Also any books on good writing.
@JohannaMakesGames There's also the ever-classic "The Design of Everyday Things" by Don Norman.