The Book of Dead Days by Marcus Sedgwick

The dead days. These are the days between Christmas and New Years. Days when spirits roam and magic prevails. And in a European city in the late 18th century, Valerian is desparate. He is running out of time. With his servant, a young boy with no name, and Boy's friend Willow, they attempt to find the book that will save Valerian. The search will not be easy. They will come upon strange and mysterious sights along the way. Through graveyards and tunnels under the city, the three must find the book before the end of the dead days. Since, the end of the dead days signal the end of Valerian.