I especially liked the recipes on using plugins in different browsers (Safari, Chrome, Opera, and Internet Explorer are covered) to simulate different devices, browsers, and screen dimensions. There is also a recipe on using virtual machine programs that simulate different devices and browsers like VirtualBox.
Benjamin LaGrone’s discussion of the current state of responsive web design today also really appealed to me; especially when he points out how even though the number of mobile device users is exploding, there are still businesses that use older desktop browsers like Internet Explorer 6. If you want to be a good responsive web designer, you must cover a wide spectrum of devices and desktop machines.
The HTML5 and CSS3 Responsive Web Design Cookbook is well-informed, but its recipes where you type code need to be spruced up.