City of Masks - Mary Hoffman
I'd seen this book around, but since it's the first in a series, I'd resisted picking it up till now...

The basic premise of City of Masks is that there exists the ability of certain individuals (the stravagante) to travel between Earth and a parallel world much like it, but different in time. Hours passing in our Earth are equal to days in Bellezza, a city much like Venice and ruled by a masked Duchess.

Our protagonist, Lucien, is a teenage boy living in our London who is undergoing chemotherapy and discovers that he has this ability, leading him to a double life in both worlds. Naturally, there are plots and counterplots galore as the life of the Duchess is threatened and Lucien's new friends hold the key to the city's future prosperity, in more ways than one.

It's an enjoyable read, with strong characters (both male and female) although I guessed at least one of the major plot-twists way earlier than I was probably meant to. The series continues in City of Stars.