City of Masks by Mary Hoffman
Lucien, a young boy living in England, is sick in bed with cancer. His father brings him a beautiful journal that looks like it is from Venice. Lucien begins to read about Venice and becomes very intriuged by the Italian city. One night, Lucien falls asleep with the journal in his hand and finds himself transported to a place very much like Venice, Italy.
In this strange city, called Talia, Lucien is healthy again. He makes friends - and enemies - with the people there and begins to like living there. Returning to Talia every night, Lucien uncovers a plot to kill the Duchessa. Meanwhile, in his own world, he seems to get better. But will these night time adventures affect his health? Will Lucien save the Duchessa of Talia?
Next book: City of Stars