Advanced Programming in the Unix Environment
From Arnold's list on for learning the Zen of Unix.
Modern Unix systems (including Linux) offer a wealth of APIs. This book provides detailed instruction, including sample code, on how to make the best use of them. It is a one-stop-shop for mastering the details of Unix programming in C.
I recommend the book because it is thorough, authoritative, and clearly written. Richard Stevens was a well-known author and consultant and his books are models of clear exposition. Steven Rago is an experienced Unix developer and author as well.