Hello! I pre-ordered the game, and have access to the Steam key, but don't know where to find the artbook and the mini-guide that comes with it.
Hello! If you pre-ordered the game on Itchio, you should have received the mini-guide on the email that is attached to your Itchio account around a week ago. (It might have landed in your spam folder though.)
If you haven't received it, please send an email to customer.niftyvisuals(at)gmail.com from that email address and we will get back to you!
As for the art book, it is not released yet, but if you have pre-ordered it, you will get a key for it over email once it is as well :D