Web Server Administration
Apache Cookbook: Solutions and Examples for Apache Administrators (2008): Rich Bowen, Ken Coar: Written by the members of the Apache Software Foundation, this book offers content in the form of guides for various problems and requirements that you will face in a job. Divided into topics like Security, Performance, SSL and others, the book deals with problems ranging from novice-level to advanced-level. For every problem addressed in the book, you will find a solution that includes short pieces of code and explanations as to why the code works, and how it works. This allows you to understand the solutions rather than just a patch-job on problems which you would have no clue about.
Apache Security (2009): Ivan Ristic: Dealing exclusively with securing Apache, this book acts as a single source for most of the information required to secure your Apache webserver. The book combines topics in securing Apache, plus various other scenarios and topics located outside the scope of Apache itself. And with more than half of the web-servers running Apache today, this book is of utmost importance for anyone delving into securing Linux and its services.
DNS Server Administration
DNS and BIND (2006): Cricket Liu, Paul Albitz: This book is the de-facto reference for any system-administrator. You can read it from cover-to-cover or keep it on your shelf for reference whenever you need it. The book starts with simple configuration and installation of BIND, one of the most used DNS servers today, and moves on to complex configurations for multiple domains and security. If your system/network is connected to the Internet, you need to read this book or refer to it for your DNS-related tasks.
Security
Linux Firewalls (2005): Steve Suehring, Robert Ziegler: This book deals with securing networks at the home-office level to full blown enterprise setups. Since Linux includes a world-class firewall system in the form of Ipfilter, Netfilter and Iptables, the book serves as a great reference for people involved with securing Linux networks. The book includes examples for creating and debugging firewall rules and moves on to things like intrusion detection, trust-level based security and reporting intrusions to network staff.
Nmap in the Enterprise: Your Guide to Network Scanning (2008): Angela Orebaugh, Becky Pinkard: To defend your own network, you must also find its weaknesses. And what better tool to start your search than nmap? The book starts off with the absolute basics; OSI layers, basic network protocols and nmap techniques. It then moves on to much more complex stuff like monitoring techniques and advanced scanning techniques. Definitely recommended on every sysadmin's bookshelf.
Virtualization
Running Xen: A Hands-On Guide to the Art of Virtualization (2008): by Jeanna N. Matthews, Eli M. Dow, Todd Deshane, Wenjin Hu, Jeremy Bongio, Patrick F. Wilbur, Brendan Johnson: With increasing server specs and cloud computing becoming more and more popular, virtualization has taken big leaps in the last couple of years. With the potential of reducing hardware costs and easing administration, virtualization should be looked into by every sysadmin. And at the forefront of virtualization is Xen. This book starts off with basic Xen knowledge, and moves on to large-scale deployment knowledge like pre-built system images, managing multiple VMs, designing and configuring complete Xen networks and managing guest resources. The people who have written this book know Xen inside-out and it shows.