The Caves of Steel
The first of the robot novels.
This novel introduces Elijah Baley and Daneel, a human policeman and a robot, respectively, who stay with the series for the next three books (Daneel is in the fourth, Robots and Empire, as well). Elijah comes from an Earth of the future, where humans live underground in huge Cities, and Daneel is a robot from the Spacer culture (Spacers are humans who have colonised other planets). The two, after some initial friction - due to the fact that Earthmen hardly ever encounter robots in their daily lives - get on well together.
The whole book, as well as being a science fiction novel, is also a crime story, because Daneel and Baley have to solve a crime on Earth - the murder of a Spacer who lives in an Earth colony. Spacers wield the most power throughout the universe, and this murder is very important for the Earthmen to help solve.
I won't spoil the ending, but this is a very riveting story, especially with the crime element thrown in, and it extends and adds to the previous Robot short stories from The Complete Robot, putting them into a full novel context. A good read.
'One of the classic presentations of the womb-city, metropolis as mother, which has haunted imaginations ever since ... The Caves of Steel and The Naked Sun are the best books Asimov ever wrote.'