The Taj Mahal—which the book is about—is, of course, made of white marble.
But what's up with a marble sky?
Well, the Taj Mahal is huge and majestic. You've seen those huge marble domes, right? Now, if you were to stand inside the world's most famous mausoleum, you might look up and think you were seeing a marble sky. Well, that's pretty appropriate: this is a book about the people who built the Taj Mahal, and the whole thing is named after what it would feel like to be in the Taj Mahal.