From the Mixed-Up Files Of Mrs Basil E Frankweiler by E L Konigsburg

When Claudia decided to run away, she planned very carefully. She would be gone just long enough to teach her parents a lesson in Claudia appreciation. And she would go in comfort-she would live at the Metropolitan Museum of Art. She saved her money, and she invited her brother Jamie to go, mostly because be was a miser and would have money.
Claudia was a good organizer and Jamie bad some ideas, too; so the two took up residence at the museum right on schedule. But once the fun of settling in was over, Claudia had two unexpected problems: She felt just the same, and she wanted to feel different; and she found a statue at the Museum so beautiful she could not go home until she bad discovered its maker, a question that baffled the experts, too.
The former owner of the statue was Mrs. Basil E. Frankweiler. Without her-well, without her, Claudia might never have found a way to go home.

When Claudia and Jamie plan to run away from home, they decide that the Metropolitan Museum of Art would be a very comfortable place to live. So they settle in and soon find themselves in the middle of a controversy over the authenticity of a new statue.