The problem is that Ginter has had the book listed that way for quite some time and has bypassed it with a number of the actual releases, including the YF-23 and XB-46 books. So no guarantees that it will appear anytime soon.
I wish he'd also do a book on the XF-12 Rainbow, but the release of Mike Machat's book a few years ago makes that a bit less of a priority. I'd also like to see books on the C-74 (without the C-124), the XC-120, the XCG-18/YC-122 and the C-132.