The Hand of Oberon (serialized in Galaxy May, July, and September 1976.)

We begin in a place that is the true Amber - source to Amber, as Amber is the source from which all other realities reflect. Corwin, Random, and Ganelon were brought here for a purpose: "to determine how the Pattern was damaged and what might be done to repair it." The Black Road and its effects were caused by spilling the blood of a Son of Amber on the Primal Pattern. They find a clue which may indicate a son of Random had been lured here… (and killed?) Corwin keeps looking for reasons, clues…and get answers: like how Corwin came to be on the shadow Earth without his memory, how Brand is not only alive, but plans to destroy the Pattern and Amber and all reality reflecting from them - replacing it with his own design. And while Oberon is still missing, his presence is becoming stronger. One of my favorite parts of this novel was discovering that Roger Zelazny had written himself into his own series: as a guard named Roger, writing "a philosophical romance shot through with elements of horror and morbidity."