This book isn't focused on recent trends or properties specific to a certain field, and thus applies to all design. It provides a broad view and allows focusing on the fundamental goals of the user experience, rather than being limited in scope.
Much of the book is dedicated to getting the main point across (don't micromanage, let people do their job). This can make it feel repetitive and dull at times.
This book was originally published in the 70's, and it's most recent edition published in the 90's. Despite it's age, much of the topics remain relevant today. The principles remain timeless, though the explanations can be dated.
The underlying principles of the book are still, in most cases, relevant today. However many of the examples are dated (such as references to older methodologies).
The author discusses how haste and unrealistic schedules will negatively impact a project. Causing the deadline to extend well past what it could have, had a realistic time frame been put in place initially.
This is a large book at 680 pages, so having the lessons summed up is quite beneficial. 27 "Best Practices" are discussed, including the pros and cons of them.