Listmania!
Destination UNIX
By an Amazon.com customer
Linux Administration HandbookLinux Administration Handbook by Evi Nemeth
Buy used from: $0.01
The first step is very important.
Learning the bash Shell (In a Nutshell (O'Reilly))Learning the bash Shell (In a Nutshell (O'Reilly)) by Cameron Newham
Buy new: $23.07 / Used from: $16.78
Shell is important, learn to use it
Linux Command Line and Shell Scripting BibleLinux Command Line and Shell Scripting Bible by Richard Blum
Buy new: $31.49 / Used from: $28.62
Learn command line
C Programming Language (2nd Edition)C Programming Language (2nd Edition) by Brian W. Kernighan
Buy new: $46.42 / Used from: $19.87
You can't do something interesting without C language
The Art of UNIX ProgrammingThe Art of UNIX Programming by Eric S. Raymond
Buy new: $28.19 / Used from: $14.42
Unix philosophy. Answers to yours "Why?" questions.
Learning GNU Emacs, Third EditionLearning GNU Emacs, Third Edition by Debra Cameron
Buy new: $26.37 / Used from: $18.79
you need it not just for editing
Understanding UNIX/LINUX  Programming: A Guide to Theory and PracticeUnderstanding UNIX/LINUX Programming: A Guide to Theory and Practice by Bruce Molay
Buy new: $72.46 / Used from: $56.70
Begin to understand Unix internals
The Complete FreeBSD: Documentation from the SourceThe Complete FreeBSD: Documentation from the Source by Greg Lehey
Buy new: $34.64 / Used from: $2.87
Something solid - BSD
Building Firewalls with OpenBSD and PF, 2nd EditionBuilding Firewalls with OpenBSD and PF, 2nd Edition by Jacek Artymiak
Buy new: $44.99 / Used from: $41.98
Practical and useful
The Debian System: Concepts and TechniquesThe Debian System: Concepts and Techniques by Martin F Krafft
Buy used from: $23.95
Something solid - another point of view
Linux Firewalls (3rd Edition)Linux Firewalls (3rd Edition) by Steve Suehring
Buy new: $37.11 / Used from: $13.00
Designing and Implementing Linux Firewalls with QoS using netfilter, iproute2, NAT and L7-filterDesigning and Implementing Linux Firewalls with QoS using netfilter, iproute2, NAT and L7-filter by Lucian Gheorghe
Buy new: $31.57 / Used from: $29.99
Advanced Programming in the UNIX(R) Environment (2nd Edition)Advanced Programming in the UNIX(R) Environment (2nd Edition) by W. Richard Stevens
Buy new: $62.39 / Used from: $44.00
Oh my master
The Design and Implementation of the FreeBSD Operating SystemThe Design and Implementation of the FreeBSD Operating System by Marshall Kirk McKusick
Buy new: $55.99 / Used from: $39.99
Nirvana!