The standards for usability and interaction design for web site design and those also for software interaction design are well known. While not everyone uses those standards, or uses them correctly, there is a large body of knowledge, best practice, and proven results in those two large fields, and a good education system for teaching professionals "how to."
For the relatively new field of web application design, however, what we mostly have are designers reusing the old rules in a new platform: no new best practices yet exist. This book, written by designers with a significant contribution to web-based application design, deliver both a thorough treatment of the subject for many different kinds of applications, plus a quick reference for designers looking for some fast design solutions and opportunities to enhance the web application experience.