Roger Zelazny's Shadows of Amber

Oberon, newly-crowned King of Amber, finds himself in the middle of deadly political machinations, as his father tries to turn him into a puppet ruler. Meanwhile, rumors abound of a Shadow Amber in the sea, where a distorted version of Oberon sits on an onyx throne. To make matters worse, Oberon's sister is trying to marry him off to a grasping would-be queen, at least two siblings are out for his blood, and the entire Shadow-universe is starting to unravel. What's a new king to do? Seek help from an unlikely new ally!