This is what I found on Facebook in case you were interested this was posted on march 19, 2020
"The Codex - Where is it and when will it be out?
The short answer is, it’s still being written. Eileen plans to turn it in this year, but she’s unsure what publication date it’ll be given.
Thank you for being amazingly patient. She understands how frustrating it is to wait for a book, especially a finale!
For the most up-to-date info, consider visiting her personal Facebook page, Eileen Wilks.
BREATHE. Be kind. We’ll all get there together."
Don't know if that helped but was honestly all I found.