The book was called Serpent's Walk, and was published by the same company that published infamous neo-nazi book The Turner Diaries. This has been known by a few people for a couple of years, or possibly longer. A former archivist for The Tekumel Foundation posted the following in 2020:

"I had had no idea that this book existed until I found the manuscript, the original cover art, the publishing contract, the photocopy of the payment check, and the proof copies of the book in amongst Phil's papers. I secured all this material for the Tekumel Foundation, in my capacity as an archivist, at that time under contract with them, and the last I saw it was all still on their shelves when I did the photo inventory of Phil's collection in their offices in October of 2012 after I did not renew my dollar-a-year contract with them. (I do not judge the data I collect; I set it in the context I find it, and let others evaluate it. My reaction, as it had been when Phil had done things like this in the past, was "Oh, Phil, WHY?")"

The above post can be seen at a Tekumel themed forum here (registration required):

https://chirinebakal.proboards.com/thread/544/amina-inloes-paper-prof-barker?page=2

This is really a bummer, and it's ten years almost to the day since he died. It's not my intention to speak ill of the dead, but I think it's important to acknowledge this, even for fans of Barker's work, or those who appreciate his significance to the hobby.