HTML / XHTML
http://www.w3c.org/MarkUp/
HTML & XHTML: The Definitive Guide by C. Musciano, et al (O'Reilly Press)

Webmaster
How to set up and maintain a web site by L. Stein (Addison Wesley)
Webmaster in a Nutshell by S. Spainhour, et al (O'Reilly Press)

Dreamweaver
http://www.macromedia.com/support/dreamweaver/
Dreamweaver 4: Hands on Training by L. Weinman et al

JavaScript
http://www.javascript.com
JavaScript: The Definitive Guide by D. Flanagan (O'Reilly Press)
Beginning JavaScript by P. Wilton (Wrox Publishing)

Cascading Style Sheets
http://www.w3.org/Style/CSS/
DHTML and CSS for the World Wide Web by J. Teague (Peachpit)

PHP
http://www.php.net
http://www.phpbuilder.com
PHP Black Book by P. Moulding (Coriolis)
Teach yourself PHP in 24 Hours by M. Zandstra (Sams)

MySQL
http://www.mysql.com
MySQL by P. DuBois (New Riders)
MySQL by M. Kofler (AP)

Other Programming
http://java.sun.com (Java/J2EE)
http://www.microsoft.com/net/ (.NET/C#)
http://www.cuj.com (C++)