What do several hundreds of maps in the Iibrary have in common? How does Chang explain this? (Chapter Five)

The thing the several hundred maps have in common is the fact the Shangri-La couldn't be found on any of them. The reason? All Chang would say was that there was a very good reason.

Mallinson, who had picked up a book, interrupted: “Here’s something for your studious life, Conway. It’s a map of the country.”

“We have a collection of several hundreds,” said Chang. “They are all open to your inspection, but perhaps I can save you trouble in one respect. You will not find Shangri–La marked on any.”

“Curious,” Conway made comment. “I wonder why?”

“There is a very good reason, but I am afraid that is all I can say.”