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O'Reilly have been producing excellent technical books for years and are a valuable source of reference and training. I have broken down many of them into their respective categories.
UNIX Basics - includes UNIX in a Nutshell, sed & awk, Learning the UNIX Operating System, Learning the vi Editor, Learning the Korn Shell and UNIX Power Tools.
Advanced UNIX - includes scripting in Perl, awk, expect, tcl/tk; Red Hat Linux; NFS; NIS; printing; Samba; sendmail etc.
Oracle - includes Advanced Oracle, PL/SQL Programming, Building Oracle XML Applications, Oracle Database Administration: The Essential Reference, Oracle Essentials: Oracle8 & Oracle8i, Oracle Performance Tuning, Oracle Scripts, Oracle Security and Oracle SQL*Plus.
Internet Technology - includes Apache, Building Internet Firewalls, Building Linux Clusters, HTTP, mod_perl and Writing Apache Modules with Perl and C.
NT/UNIX - includes The Complete Windows NT & UNIX System Administration Pack, DHCP, NT System Administration, Perl on Win32 Systems, IMAP, Cisco Routers, the Windows Registry, Securing Windows NT/2000 Servers for the Internet, Windows 2000 Quick Fixes, Windows 98 Annoyances and Windows NT TCP/IP Network Administration Word 2000 in a Nutshell
Security - includes Computer Security Basics and Cracking DES