The Robots of Dawn (Robot, Book 4)

On Aurora, the first and greatest of the Spacer planets, Elijah Baley and R. Daneel Olivaw investigate yet another seemingly impossible crime - this time, a roboticide. Someone has destroyed the positronic mind of R. Jander Panell, a humanoid twin to Daneel. His creator, the master roboticist Han Fastolfe, denies all involvement. So does Gladia Delmarre, the robot's owner. And lover. Working in the heart of Spacer politics and civilization, Baley and Daneel soon realize that their decisions will have profound consequences not only on relations between Earth and the Outer Worlds, but on mankind's place in the galaxy.