Linux Network Administrator's Guide (2009): Tony Bautts, Terry Dawson, Gregor Purdy: Rounding off this article is a small but incredibly useful book for administering a Linux network and various protocols and daemons. This book takes off from the basics like TCP/IP networking, firewalls and wireless networking, and provides information about various protocols and daemons like OpenSSH, IMAP, OpenLDAP, IPv6 and so on. The book emerged from The Linux Documentation Project and is therefore well suited to any Linux administrator delving into networking and network-services administration.
Linux Command Line and Shell Scripting Bible (2008): Richard Blum: No system administrator's knowledge-base can be complete without knowledge of at least one scripting language for day-to-day tasks. It can be Python, Perl or even Bashscript. This bible helps you move off the heavy graphical user interface and teaches you how to love the command-line. From the absolute basics to complex things like connecting to and managing databases, monitoring applications and teaching character parsing, this book has it all. Also learn how to use one of the many other shells available in Linux like ash, tcsh, korn, zsh and others. 5/5 for this one!
Python for Unix and Linux System Administration (2008): Noah Gift, Jeremy Jones: If you're looking for specific knowledge, don't look any further than Python. It is one of the most popular scripting languages for Linux sysadmins and with its set of capabilities, you can script just about anything you need. Learn how to manage files, interact with SNMP and monitor large clusters of machines, manage databases remotely and lots more. The only gripe with this book is that it has a few spelling errors, but that's about it.
At the end of the day, nothing beats relevant on-the-job experience of troubleshooting and setting up various technologies and solutions. These books mentioned above are what will come to your rescue when you're caught unawares with no knowledge of certain technologies and methodologies. While the books listed in this article might not always be 1000-paged bibles with all the information under the sun, they're all incredibly useful for most purposes and scenarios that you will face in a Linux administration job environment. In case of the relevant information not being present in these books, it is recommended to refer to official documentation, online forums, mailing lists and IRC channels.