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Learn How to Design Web Sites
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Head First HTML with CSS & XHTMLHead First HTML with CSS & XHTML by Elisabeth Freeman
Buy new: $26.39 / Used from: $17.81
Definitely the place to start, very easy to follow step-by-step.
Web Design in a Nutshell: A Desktop Quick Reference (In a Nutshell (O'Reilly))Web Design in a Nutshell: A Desktop Quick Reference (In a Nutshell (O'Reilly)) by Jennifer Niederst
Buy new: $23.09 / Used from: $7.94
Read it front to back, or just look up what you need...either way, keep it close by!
Prioritizing Web UsabilityPrioritizing Web Usability by Jakob Nielsen
Buy new: $27.94 / Used from: $19.00
Good reminders and examples throughout. "Form follows function..."
CSS Mastery: Advanced Web Standards SolutionsCSS Mastery: Advanced Web Standards Solutions by Andy Budd
Buy new: $22.04 / Used from: $4.97
REALLY figure out how to lay out your pages. "Function follows form..."
The Zen of CSS Design: Visual Enlightenment for the WebThe Zen of CSS Design: Visual Enlightenment for the Web by Dave Shea
Buy new: $29.69 / Used from: $15.99
See what's possible with all your new-found CSSgoodness.
The Non-Designer's Design BookThe Non-Designer's Design Book by Robin Williams
Buy new: $19.79 / Used from: $0.60
If good design ideas don't just "pop" into your head on a regular basis.
The CSS Anthology: 101 Essential Tips, Tricks, and HacksThe CSS Anthology: 101 Essential Tips, Tricks, and Hacks by Rachel Andrew
Buy used from: $2.00
Another really good CSS guidebook.
Defensive Design for the Web: How to improve error messages, help, forms, and other crisis pointsDefensive Design for the Web: How to improve error messages, help, forms, and other crisis points by Matthew Linderman
Buy new: $19.79 / Used from: $7.07
What works, what doesn't...and why.
Designing With Web StandardsDesigning With Web Standards by Jeffrey Zeldman
Buy new: $25.35 / Used from: $0.49
Find out why Web design standards matter and what they mean to your site.