The Design of Depravity was the first book I did for the game, and is currently in its 3rd edition. It is a 32 page book about the design of the game. I talk about the humble beginnings of the game, and how my method has evolved over time.
There is exclusive art and there is also a deeper look behind the scenes, from my process to the story, to the render.
It is not sold separately but it goes on special from time to time (INCLUDING NOW) which means it is a free reward for subscribers at the $10 tier or above.
Cheers,
Dante