The Linux Programming Interface: A Linux and UNIX System Programming Handbook
Are you a Linux or UNIX programmer? Get this book. Do you know a Linux or UNIX programmer and want to give him (her? ;-)) a gift? Get this book. I thought it was a stack of manpages, which would already have been great, but this book is the true successor to the historic Stevens works on UNIX. If you think you don't need this book since you know everything already, that's what I thought too, and I was wrong. Even if you won't read it, the 1552 pages will look really good on your desk.
So go get the book already.