HTML & XHTML The Definitive Guide
The popular reference manual.
O'Reilly & Associates
Chuck Musciano & Bill Kennedy

KornShell Programming Tutorial
A complete and detailed manual for the beginner or long time script writer. We're on our third copy.
Addison-Wesley Publishing
Barry Rosenberg

The C Programming Language
Written by the language architects, this reference now encompasses the ANSI standard.
Prentice Hall
Brian Kernighan & Dennis Ritchie

The C++ Programming Language
Written by the language architect, this reference includes much of the proposed ANSI standard.
Prentice Hall
Bjarne Stroustrup

Learning Perl / Programming Perl
The O'Reilly guides written by the experts and language architects.
O'Reilly & Associates
Randel Schwartz, Tom Phoenix & brian d foy
Larry Wall, Tom Christiansen & Jon Orwant

Internetworking with TCP/IP Vol. I: Principles, Protocols, and Architecture
A practical guide to designing application software for the Internet environment.
Prentice Hall
Douglas Comer

The UNIX Programming Environment
A good overview of UNIX including the filesystem, shells, standard I/O and underlying system calls.
Prentice Hall
Brian Kernighan & Rob Pike

Essential System Administration
Practical help for UNIX System Administrators.
O'Reilly & Associates
AEleen Frisch

Refactoring: Improving the Design of Existing Code
The premier manual for improving existing code.
Martin Fowler

Design Patterns
A modern classic in the literature of object oriented development.
The Gang of Four