They're about a group of people called the Stravaganti who travel from our world and time to Talia, which is a place that seems to be the parallel to Renaissance Italy. Each city state in Talia has it's own Italian equivelent. An example is that Bellezza is like Venice. Some people from Talia are Stravaganti and can travel from their world to ours.

So far the books that are out are:
City of Masks
City of Stars
City of Flowers
City of Secrets
City of Ships