Midnight Is a Place — Joan Aiken

Lucas Bell is lonely and miserable at Midnight Court, a vast, brooding house owned by his intolerable guardian, Sir Randolph Grimsby. When a mysterious carriage brings a visitor to the house, Lucas hopes he's found a friend at last. But the newcomer, Anna Marie, is unfriendly and spoiled -- and French. Just when Lucas thinks things can't get any worse, disastrous circumstances force him and Anna Marie, parentless and penniless, into the dark and unfriendly streets of Blastburn.