Sara Horowitz (founder of The Freelancer’s Union) writes The Freelancer’s Bible for freelancers of all stripes. If you’re a freelancer, you need to pick up a copy.
If you get excited by clever CSS techniques, semantic markup, and different ways you can use APIs to interact with users, The Modern Web is the book for you.
If you’re interested in developing WordPress plugins, then do yourself a favor right now an invest in a copy of Professional WordPress Plugin Development.
Professional WordPress Design and Development is an essential volume for anyone who works with the content management system in a professional capacity.
Jeremy Keith’s HTML5 for Web Designers is a short, breezy primer on HTML5 for designers and developers who want to learn a little more before jumping in.