Although the C programming language was developed in the early 70's, there was no description of it published for several years, until the famous K&R. K&R was the summary for Brian Kernighan and Dennis Ritchie the authors of "The C programming Language" which is still considered as the basic reference book for any programmer wishing to learn basis of the C language.
First edition appeared in 1978. It was the first official definition of the language and marked the real expansion of the Unix Operating System and its main language near professional and amateur users.
About this book, Dennis Ritchie wrote:
In 1978 Brian Kernighan and I published The C Programming Language [Kernighan 78]. Although it did not describe some additions that soon became common, this book served as the language reference until a formal standard was adopted more than ten years later. Although we worked closely together on this book, there was a clear division of labor: Kernighan wrote almost all the expository material, while I was responsible for the appendix containing the reference manual and the chapter on interfacing with the Unix system.
A second edition appeared about ten years later. It was completely rewritten with additional examples and problem sets to clarify the implementation of difficult language constructs.