Essential.
If you think about using C language on GNU/Linux, you need this.
You can always access the man pages of every function, but this book will teach you how the system work, why things are this way, how it was in early UNIXES...

This is the new *nix bible. I use to rely heavily on the Stevens books, but now all I need is this. I tend to perform some lower level development in my work and this has been an invaluable resource for me. It is always on my desk. My only hang up on this was the lack of a good debugger discussion, but there are plenty of other resources for that!

It's huge, heavy, and contains all the syscall incantations. Everything the kernel can do for you is in here. This will also answer pretty much any question you have about Linux internals (how is shared memory implemented? does writing to disk block your process? etc). A fantastic book.

TLPI is a book that will keep on giving. I received this book as a sophomore in college and excitedly read the first few chapters page for page. Kerrisk does an excellent job of writing in a way that exposes the way Linux APIs were meant to be used. There is a consistent and clear, though not too forceful, exposure of themes. You'll find that after a few months of working with this book the underlying Linux operating system calls become familiar in a way that upon running into a new problem you can almost guess what the system calls to get at the library you'll need look like.

This book is excellent, a complete UNIX/LINUX system programming reference! Let me preface this review with the comment that I use PC-BSD 10.1.1 p20, Solaris 11.2, and Ubuntu 14.04.2 LTS as my platforms. I have only one suggestion for those programmers that want to use this book on systems other than LINUX.
If you use the make file at the top level download directory of the book or dist source code, it is successful in compiling all of the source in each chapter on a LINUX system at one time. That's what happened on my Ubuntu 14.04.2 LTS, as of the tarball available at the book website on 4/24/15. But trying to compile the source on both PC-BSD 10.1.1 p20 or Solaris 11.2 proved to be an insurmountable task for me. If you follow the author's excellent instructions for compiling on systems other than LINUX, you may have better luck than me.
All in all, a very complete SUSv4/ POSIX -compliant reference, and well organized too!
Update 9/27/2015- Successfully compiled the source tarball on Ubuntu 15.04 using Method B in the BUILDING.html file at the book website. Had to apt-get gcc on my server edition first. Then went to each sub-directory of interest and used make to compile. Beautiful!