Listmania!
Linux must have books
Linux Kernel Development (Novell Press)Linux Kernel Development (Novell Press) by Robert Love
Buy new: £15.99 / Used from: £4.90
Ignore the title, even if you are just an system admin you need to read this book
Linux Administration HandbookLinux Administration Handbook by Evi Nemeth
Buy new: £17.99 / Used from: £23.25
If it not covered in here you don't need to know about it
Linux Security CookbookLinux Security Cookbook by Daniel Barrett
Buy new: £18.71 / Used from: £21.09
'cos there are bad guys out there
Applied Cryptography: Protocols, Algorithms and Source Code in CApplied Cryptography: Protocols, Algorithms and Source Code in C by Bruce Schneier
Buy new: £26.15 / Used from: £18.75
The book "they" didn't want published. Hard going but interesting
Python CookbookPython Cookbook by Alex Martelli
Buy new: £21.11 / Used from: £17.61
You don't still use Perl do you?
Programming Perl: There's More Than One Way To Do ItProgramming Perl: There's More Than One Way To Do It by Larry Wall
Buy new: £21.51 / Used from: £10.80
In case you need to debug old Perl code
Optimizing Linux Performance: A Hands-On Guide to Linux Performance Tools (Hewlett-Packard Professional Books)Optimizing Linux Performance: A Hands-On Guide to Linux Performance Tools (Hewlett-Packard Professional Books) by Phillip G. Ezolt
Buy used from: £21.67
Good, if a bit basic
Performance Tuning for Linux ServersPerformance Tuning for Linux Servers by Sandra K. Johnson PH.D.
Buy used from: £39.95
Slightly out of date, but worth having nevertheles
TCP/IP Illustrated: Protocols v. 1: The Protocols (APC)TCP/IP Illustrated: Protocols v. 1: The Protocols (APC) by W. Richard Stevens
Buy new: £28.96 / Used from: £19.99
You need this book, a classic