I remember the story took place in a fantasy world that could be accessed during certain phases of the moon. The main character follows a girl into it. In this world, there are certain people born with magical powers indicated by their eyes which constantly change in color. Girls are known as Chanters. Boys are known as Warlocks. This world also had a feudal system and tournaments (one chapter covers the main character participating in an archery contest). Knights ride large birds, such as chickens and hawks. A certain type of item known as a Kanarki Flower (sp?) is prized by the nobility (even going to wars over it...a plot point later on).
This world has gained several inventions and cultural relics from our world through people from our world (e.g. mills, "Greensleaves"). In particular, by a genius of an offworlder (who I think was a mason?) and following the clues left behind by this person become crucial to the plot.
The primary villain (a Duke/Count, I think) orchestrates a civil war and plans for this by secretly capturing and killing off all Chanters and Warlocks that oppose him. Final battle takes place in a room in a Duke/Count's Palace filled with Kanarki Flowers casting off multiple reflections of him. Main character beats this illusion by shooting down the flowers (and no, this is not a video game) and eventually revealing the Duke/Count so that another character could fight him.
It's painful to know so many details and not remember the name. Any help in finding this again would be greatly appreciated.