A.1. HTML and JavaScript
This book assumes that you are somewhat familiar with HTML and JavaScript. To follow along, you can probably get away with not knowing the latter, but a basic understanding of HTML is a must, and acquiring some knowledge of JavaScript is very beneficial as well.
A.1.1. HTML Links
W3Schools' HTML Tutorial: http://www.w3schools.com/html/DEFAULT.asp
HTML Primer: http://www.htmlgoodies.com/primers/html/
HTML Code Tutorial: http://www.htmlcodetutorial.com
HTML on Wikipedia: http://en.wikipedia.org/wiki/HTML
XHTML on Wikipedia: http://en.wikipedia.org/wiki/XHTML
HTML 4.01 Specification: http://www.w3.org/TR/REC-html40/
XHTML 1.0 Specification: http://www.w3.org/TR/xhtml1/
W3C's Markup Validator: http://validator.w3.org
W3Schools' CSS Tutorial: http://www.w3schools.com/css/
A.1.2. JavaScript Links
Mozilla's JavaScript guides: https://developer.mozilla.org/en/JavaScript
W3Schools' JavaScript Tutorial: http://www.w3schools.com/JS/default.asp
JavaScript on Wikipedia: http://en.wikipedia.org/wiki/JavaScript
The JavaScript Programming Language (four parts): http://video.yahoo.com/watch/111593/1710507
JavaScript — The Good Parts: http://video.yahoo.com/watch/630959/2974197
The Theory of DOM (three parts): http://video.yahoo.com/watch/111582/992708
Welcome to Firebug 1.0: http://video.yahoo.com/watch/111597/1755924
A.1.3. Common Ajax Libraries and Frameworks
Prototype: http://www.prototypejs.org
script.aculo.us: http://script.aculo.us
jQuery: http://jquery.com
dojo: http://dojotoolkit.org
Ext JS: http://extjs.com
MooTools: http://mootools.net