- UNIX in a Nutshell
- UNIX Power Tools
- UNIX and Linux System Administration Handbook
- From Bash to Z Shell: Conquering the Command Line
- Working with UNIX Processes
- Advanced Programming Unix Environment(http://www.amazon.com/Advanced-Programming-UNIX-Environment-2nd/dp/0201433079/ref=sr_1_1?ie=UTF8&qid=1329768694&sr=8-1)
- The Art of UNIX Programming - I like the first chapter (available freely on Amazon) that talks about the UNIX Philosophy as it is applicable to software development in general. The first chapter is definitely useful for people with all backgrounds. I cannot attest for the remainder as I myself have not read through it. You can find the electronic version of the whole book here. H/T Ryan Briones
@tundal45 APUE is definitely not for the beginner. It's a book aimed at reasonably experienced C programmers. Once you have the basics of UNIX, it's a great way of getting down into the details so that you understand why things work the way they do. Over the course of my career, my deep knowledge of UNIX has been invaluable for debugging and performance purposes.