The Hand of Oberon by Roger Zelazny

In this installment, Corwin finally begins to figure out much of what's been going on. At the primal Pattern, he and Random and Ganelon discover that the blood of Martin, Random's son, has been shed in the middle of the Pattern, causing damage, with the effects spreading out through Shadow, creating the Black Road which leads from the Courts of Chaos to Amber. Random sets off with Benedict to try to find Martin and determine his fate, as Benedict was one of the last people to hear news of Martin, via some mutual friends in Shadow.

Corwin uses an etching in his old cell, carved by Dworkin, the ancient family wizard to visit Dworkin where Oberon has confined him. He discovers that Dworkin was a rebel son of Chaos who found the Jewel of Judgement on the neck of a unicorn, and used the pattern in the Jewel to inscribe the Pattern, which created Amber and all of its Shadows, unsettling the balance between chaos and order. He is also stunned to learn that Dworkin is Oberon's sire, and thus, Corwin and his siblings' grandfather. Dworkin has gone slightly around the bend, after the damage to the Pattern caused parallel damage in his own mind, and believes the only way out is to destroy the Pattern, while Oberon, when he was last seen, believes there is a way to repair it, using the Jewel as a template.

Corwin journeys to the shadow Earth, where last he left the Jewel, after the attempt on  his life. While he doesn't discover the Jewel, he finds out through his friend and lawyer, Bill Roth, a few more things about the accident that seemed to start off this whole journey. His brother, Brand, seems to be at the bottom of the mess, and he has managed to steal the Jewel to implement his own plans for Amber.

There's a great synopsis of "what has gone before" in chapter 2 of this book, providing a nice refresher for those who wait too long between reading these. I believe the Amber chronicles were originally serialized in one of the pulps, then collected into five paperbacks, and later merged into two volumes by the SF Book Club, so early readers could have easily been left behind.

Corwin also manages to get an entirely different version of the story of the cabals vying for the throne from Julian and Fiona, Brand's former allies. It was they who had imprisoned Brand, fearing his plans for Amber were not really benign. The novel ends with a big reveal of the whereabouts of Oberon, that in retrospect seems almost obvious - the clues were there all along.

More good stuff from a master of fantasy.