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I've read this book about 10 years ago, an I think it's part of a series of books. It's high fantasy, and I remember that the protagonist is a commoner, perhaps female (but I'm not sure). The main feature I do remember is that when dragons fly, their shadow causes forgetfulness to people it passes over. There is an island/continent separated by sea from the main place where the story is told that dragons live there and the people there are like zombies, completely without memory. The main place (continent) rarely sees dragons and they fear them. I think that during the book the other island/continent is starting invade the main place/continent by stealth (i.e. infiltrating a few at a time).