The Naked Sun
The second of the robot novels.
In this book Elijah Baley and Daneel go to the planet Solaria, one of the planets which has been colonised by the Spacers, to solve a murder. The Solarians have some very strange customs - such as not allowing anyone to 'see' them, and this only makes the partners' job more difficult. The difficulty of the job is compounded by the fact that Baley is not used to the world outside his underground Cities, and the 'naked sun' causes him much fear. As always, Daneel is at his side, to help him along - the two have become firm friends by now.
The book is another detective story, similar in style to the first, but the book explains important concepts and ideas about the Spacer world which are important to understand for later books. If you liked The Caves of Steel, you'll like this book.
'The Caves of Steel and The Naked Sun are the classic SF-detective story hybrids ... they have entertained countless readers with the exploits of plainclothesman Elijah Baley'