My wife and partner in everything, Hiroko, is who introduced me to yokai culture. She has been a fan of Japanese ghost stories and spooky tales and yokai since her childhood, and while I'd known about them generally from manga and anime it was her who took me down the rabbit hole of their history!
We wanted to translate Sekien from virtually the moment we first read it together. His yokai encyclopedias are such seminal works, the literal guidebooks for so much monster and character culture to come out of Japan. Translating them was INCREDIBLY tricky and time consuming. We were fortunate to have "translations" of the handwriting into modern fonts that have been published in Japan over the years. But not all of it has been rendered this way, so we had to turn to specialists to decipher several parts. We worked closely with our editor Matt Treyvaud who is a whiz with Classical Japanese, and the three of us spent weeks puzzling out difficult parts together, sometimes with advice from professors or other academics.
Here's the finished product for those interested: https://www.amazon.com/gp/product/0486800350/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0486800350&linkCode=as2&tag=httpaltjapant-20
I know this is six months old, but I just came across this post tonight and thought you might be interested in this book:
Japandemonium Illustrated: The Yokai Encyclopedias of Toriyama Sekien https://www.amazon.com/dp/0486800350/ref=cm_sw_r_apan_glt_i_BCBN7JZ5M06E64G7VWGE?_encoding=UTF8&psc=1
Have you tried looking at the "Japanese monster manual?" This guy, Toriyama Sekien, decided he'd document all the folklore monsters of Japan. If you've ever seen images of old ink drawings of asian monsters like ogres or goblins, it was probably this guy.
Japandemonium - English translation of Toriyama Sekien's illustrated books on Yokai.
There's also Night Parade of One Hundred Demons which is like an updated version which reads exactly like a monster manual minus game stuff.
Their very good if you need some different inspirations to break you out of a writer's block of typical Tolkien beasts and monsters if you find yourself in one.
e- added toriyama link