The Stravaganza series by Mary Hoffman
I recently read the first three books in this superb series: City of Masks, City of Stars, and City of Flowers.
The series opener, City of Masks, is the story of fifteen-year-old Lucien Mulholland whose father gives him a beautiful journal to help him survive his chemotherapy treatments. When Lucien falls asleep holding the book, he travels to Bellezza, a Venice-like city located in a fantastical version 16th Century Italy. In Bellezza he becomes Luciano and learns that he is part of the brotherhood of the stravagante, wanderers between two worlds dedicated to protecting the separation of and balance between the worlds.
As Luciano explores Bellezza by night, Lucien's body becomes stronger. That physical change, however, is not why he was called to Bellezza. It is only when a plot to assassinate the Duchessa, Bellezza's beloved ruler, is uncovered, that Luciano begins to see what his true role will be.
City of Masks has it all - romance, intrigue, historical detail, subtle but strong magic, and a singular concept. Despite the fact that City of Masks stands alone and comes to a satisfying conclusion, you can't help but want to read more about Hoffman's fantastic world.
The Stravaganza books are written for teens, but they will also captivate adult readers (especially those who tend to like fantasy). The books are relatively quick reads, but they are meaty enough that you don't want to rush through them and that you feel satiated upon finishing them. Hoffman's Talia is an Italy like our own 16th Century Italy in many ways though there are unique differences, some obvious and others subtle.
The books will hold up to (and in many ways invite) multiple readings. I'm eagerly awaiting the fourth book in the series, City of Secrets, which will be published sometime this year.