At the first look, it seems to be outdated. The topics, which this book cover, are already educated by all most all people, and are covered by some program such as gulp or grunt that handle all trouble some detail for developer. However, the real good point of this book, at least for me, is that it teach how to analyze website so that I can improve the performance. I could improve the performance of my program by walking through this book.
If you are into front-end development, then this book is a good resource. This book explains how to improve your site performance (loading time) significantly. I bought this book back in 2009 and learned a lot from it. Although if you use framework, most of these optimization can be done from the framework, but I still think this book is a good read if you are into front-end development.
Also, all of the points covered in the book is mentioned in Yahoo Developer Network's Performance blog.[...]
There's also a youtube video titled "High Performance Websites and YSlow!" which explains these points in details.
I remember when I first started using the Internet. Dial ups were extremely slow at the time, so I'd type in a URL, then go make a sandwich. By the time I came back, hopefully the page would be loaded.
Today, we expect more. Often if a page takes more than a few moments to load, I don't bother. I tend to equate professional with quick. If a site doesn't load quickly or if parts of the page are slow, I naturally assume that the information provided might be as shabbily compiled. I simply move onto a different page.
High Performance Web Sites looks at how we can make our own websites load more quickly. I was surprised at how many different little things that can be done beyond optimizing graphics. Most of these things only take a few little nips and tucks and none were beyond my novice level of ability.