The Art So Far (2018)
This art book is a compilation of sketches, character designs and building designs. May contain some spoilers.
This was made as Denny's final project for art school, in which much of the series' early production took place, so take it with a grain of salt. Pretty much everything has been revamped for the 2022 release. You can get the full PDF here, or flip through some pages on the left there.
Work on this particular project spanned from September 2017 to April 2018.