The Hand of Oberon: Book Four;

The fourth in the Amber series reveals two surprising answers…

Family blood has been spilled on the Pattern that created Amber, throwing the entire kingdom in chaos. In a desperate effort to save themselves, the princes and princesses of Amber—led by Lord Corwin—search for answers. Where has their father, Oberon, has disappeared to, and what evil has created the black road that unites Amber and that multidimensional world called Shadow? Most importantly, there is a traitor in their midst.

...Who?

One of the most revered names in sf and fantasy, the incomparable Roger Zelazny was honored with numerous prizes—including six Hugo and three Nebula Awards—over the course of his legendary career. Among his more than fifty books, arguably Zelazny’s most popular literary creations were his extraordinary Amber novels.

“He was a storyteller without peer. He created worlds as colorful and exotic and memorable as any our genre has ever seen…” —George R.R. Martin