A Book Apart is at it again with their fourth title - Responsive Web Design. Their own writeup absolutely sells it:

From mobile browsers to netbooks and tablets, users are visiting your sites from an increasing array of devices and browsers. Are your designs ready? Learn how to think beyond the desktop and craft beautiful designs that anticipate and respond to your users’ needs. Ethan Marcotte will explore CSS techniques and design principles, including fluid grids, flexible images, and media queries, demonstrating how you can deliver a quality experience to your users no matter how large (or small) their display.

Even if you know this stuff already, it's likely not a terrible idea to get an alternate perspective and hear techniques that may easily become considered Best Practices.