Lud-in-the-Mist. by Hope Mirrlees

Two hundred years ago, merchants in the small country of Dorimare, weary of the capricious nature of the ruling class, cast out the aristocracy and established their own government. Believing that their former rulers' erratic behavior was caused by their neighbors in Fairyland, the mention of anything fairy quickly became taboo among Dorimare's citizens. Now, though, illicit trade of fairy fruit is on the rise in the border town of Lud-in-the-Mist and the government seems powerless to stop it.
As influential families fall victim to the maddening effects of the fruit it becomes clear that someone is working to undermine the status quo, and the Mayor needs to discover who it is before it's too late. Lud-in-the-Mist is Hope Mirrlees's third and most popular book. It has been reprinted several times, and was a part of the Fantasy Masterworks series produced by Orion Books. It has been referred to as a masterpiece of fantasy literature, with Neil Gaiman calling it "a little golden miracle of a book".