What child doesn't dream of running away to the Metropolitan Museum of Art?
This was one of my favorite books as a child. The adventure of two kids running away to New York City and hiding in a museum was fascinating to me. In addition, they get to solve a mystery, which appealed to me even more strongly.
I agree with another reviewer who says that it was the details of the book that drew her in: packing their clothes in their instrument cases, the way they spent money, the wonder that Claudia feels as she gets to experience the museum after closing time. All of these are exquisitely detailed.
As an adult re-reading this, I'm surprised by how much I enjoy it even now. I think that part of the reason is the children themselves. They're drawn as real children: they squabble, they're irrational and irritable, and they think like children do. That's what makes them so interesting.