The Other Side of the Wall by Amy Ephron

It's Christmas break and Tess and Max are in London, staying at the posh Sanborn House with their Aunt Evie. As they wait for their parents to arrive, there is an unusual snowstorm that makes the city seem as if it's caught in a snow globe. Perfect weather for an adventure in Hyde Park. But when Max, Tess, and Aunt Evie leave to search for a cab, they find a horse and carriage and driver curiously waiting for them at the curb. And that's just the beginning...

Soon Tess is charmed by a mysterious boy named Colin who lives at the hotel all year round--on the 8th floor. But Max is sure the elevator only had 7 floors the day before. And how come everyone at the hotel seems to ignore Colin? Things seem to get stranger and stranger. There's a 1920s costume party in Colin's parents' apartment. A marble that seems to be more than it appears. And a shadow that passes mysteriously by Tess and Max's hotel window.