I'd recommend Dennis Jenkins' great book "Hypersonic." It is filled with B&W and color photos, color side-views, technical drawings and a ton of information, including flight-by-flight details of the program's 199 flights. I know Ken relied on the book when he worked on his model. I have a copy, but it's back in storage in St. Paul. I have the original hardcover, and it's too darn heavy to stick in luggage when I've got a 50-pound limit. It is out in paperback now.
https://www.amazon.com/Hypersonic-St...+american+x-15
Jenkins also put out a book of X-15 photos he didn't have room for in "Hypersonic":
https://www.amazon.com/X-15-Photo-Sc...s=X-15+Jenkins
As for tail markings, one of the other peculiarities is that they would differ on the same plane; the left side of the tail might have the yellow NASA band and the right side might not. It's like modeling the Space Shuttle; to do it accurately, you almost have to pick a specific mission and model
it.