Stravaganza City of Masks

Lucien is very sick with cancer and struggles with his parents' worry every day. But each night, through a magical gift from his father, his mind is transported to an enchanting city, Bellezza, a parallel city to Venice of our world. In Bellezza, Lucien discovers that he is a Stravagante, a rare person able to travel through worlds while sleeping.
Befriended by a local girl and protected by an older Stravagante, Lucien uncovers a plot to murder the city's beloved ruler, the Duchessa. But to save the Duchessa and the city Lucien risks his only chance to return home to family and his real life.
The well-paced, thick-with-plot story will hook the reader immediately and not let go until the superb, unexpected end. City of Masks is the first in the acclaimed Stravaganza series from the gifted Mary Hoffman.