This may not be the book you're thinking of, but it might hold you over. By my favorite author Bruce Grant, romel rein construction can be found starting on page 164. I am including the whole book because he references other knots in the construction of the reins. This is a scanned version of the book so it is a little hard to read, but if you like it, I would suggest you purchase a hard copy. I've seen it available through Amazon.
ps. It is a fairly large file and could take a while to download.
Encyclopedia_of_Rawhide_and_Leather_Braiding.pdf