The title of the book is of course a throwback to Steve Souders' epitomous High Performance Web Sites: Essential Knowledge for Front-End Engineers, released a few years back by the same publisher. In much the same way it covers all aspects of performance in its chosen realm. That book gained Souders much appraise for making the web developer community at large aware of the various performance issues connected to the frontend, and how & why optimizing time was better spent there than on the backend which had previously been the prime target for such efforts. Last year Souders piggybacked on that appraise by releasing a sequel titled Even Faster Web Sites: Performance Best Practices for Web Developers (EFWS), where he - along with a group of co-authors, including Zakas - delved even deeper into frontend performance.
Definitely yes. Although a Venn diagram would show quite a bit of overlap...
* HPJS chapter 1 (Loading and Execution) is largely made up of the same content as EFWS chapter 4 (Loading Scripts without blocking)
...the books have enough diverse content, difference in tone of voice and primary focus, to make for two quite different reads.
The question of unique content, I feel, is largely moot anyway, as it is very rare to find a book containing knowledge that cannot be found elsewhere. That's not a bad thing, it's just the way of the web. When buying a programming book, you're paying for the convenience of having lots of related material collected in one place. The research behind HPJS chapter 2 (Data Access), for instance, has been detailed on Zakas' blog, just as Stoyan has already blogged a lot of what ended up in chapter 3 (DOM Scripting).
So downrating the book for being a "compilation", as one of the few not-so-positive other reviewers here does, is rather unfair and beside the point. HPJS should be judged, instead, by how well it weaves it all together, and of course by the quality of the individual chapters. In my book, it receives top scores in both of these categories.
Some co-authored books while inevitably feel rather fragmented. There are moments in EFWS when the (very) different writing styles of contributing authors gets in the way of seeing the whole picture. Similar moments arose for me while reading the semi-recent jQuery CookBook, which - while excellent - at times feels very schizophrenic. Of course co-authors need to be given some artistic leeway as to how they express themselves, but when they seem to have different takes on the main ideas behind the book, it becomes a problem.
This never happens in HPJS, which obviously has been the target of some very loving editing. Even though the different performance aspects have quite a different flavour, as does the writing of the contributing authors, you never lose the sense of context. The fact that the book stays true to its gospel - performance - is one of its biggest strengths.
There must have been innumerable temptations to mention non-performance related things that could be made to sort under a chapter's domain, but not fit inside the book as a whole. I'm sure, for example, that Steven Levithan bit his tongue while writing the (brilliant!) chapter on regexes, forcing himself not to share parts of his vast regexp knowledge that doesn't relate directly to performance. Because he and his peers withstood that temptation, HPJS is a better book.
So, to finally bottom-line this; both books have their place. EFWS and HPJS are partly speaking about the same things, but in different voices under different headlines to different people. Also, HPJS is a bloody brilliant book, and not owning it should be reason enough for ostracication from the frontend community.