City of Masks — Mary Hoffman

Lucien is very sick: he struggles just to sit up in bed, plagued with exhaustion brought on by the treatment for his cancer. But a mysterious gift from his father changes all that. Whenever Lucien holds the beautiful Italian journal, his mind is swept away to an enchanting Venice-like city called Bellezza while his earthly body sleeps peacefully.

During his many visits to the city, which become more and more real, Lucien learns that he is a Stravagante, capable of traveling between two worlds. But as his body begins to follow his mind to this other world, there is always the chance that Lucien may be caught on the other side of time and not be able to return.