I’ve used this folding technique for my book Summer as well for a book I did as part of my on-going prompt challenge, shown in this blog post. David Rosen taught me this fold. He had instructions on his website, but they (and it) have disappeared.
The photo above shows an example of the fold (an invitation to an exhibition of works by Julie Chen.)
Directions for the fold are here.
UPDATE: Click here to see other posts on the Turkish Map Fold and its variations.