The C Programming Language is a book that was written by Brian W. Kerninghan and Dennis M. Ritchie and comprises the original specification for the C Programming Language (surprised, eh?).
The book's copyright reads "(c) 1978 Bell Telephone Laboratories, Incorporated" and contains a note indicating that the book was typeset on a PDP-11/70 system running UNIX. This has become over time an interresting historical document (as well as a useful reference manual, although these days a lot of the language has changed (see ANSI C) ). Bell Telephone Laboratories became Bell Labs at some point, and is now known as Lucent Technologies.
In the book the authors lay out the basics of the language starting with simple stuff like performing mathmatical calculations and using numbers, and work their way through text handling and data structures. In the process of providing this tutorial they flesh out the features of the language as they are used in common computing tasks.
At the end of the book there is a reference section where the language is actually formally defined and enumerated. It is also interresting to note that since C is a fairly flexible and fairly low level language there are a number of ways to perform any given operation, and the example code in this book essentially establishes the accepted idiom for performing any given task (in that there are several ways you might just as well go about it, all of them equally efficient, but there is one that any C programmer will instantly recognize and understand, and all the others will cause a little bit of head scratching before their purpose is discerned).
All in all, despite being dated, it's a book that any computer programmer should be proud to own, and will most likey enjoy reading for pleasure. The sense of humor in the book is also quite geeky and enjoyable. As a couple people have pointed out, there exists also a second edition that has been updated to include the ANSI C standard which is in common use now.