If you are looking for an introduction to Unix, this is the book you're looking for. If you're looking for an introduction to ANSI C, again, this is probably the book you're looking for. If you want one book that will introduce you to both, well, this is the only book you want.
C is a high-level programming language created by Brian Kernighan and Dennis Ritchie ("K&R C") at Bell Labs. It is often considered to be the high level language closest to programming in pure assembly code. Unix was the first operating system to be implemented in a high level language, which was designed for the purpose and vice versa - The C language.