I have the book and all three versions: PSE4, PSE5 and PSE6. I think the book applies to all versions, while the tools must be installed in a different way in each version. Richard says some tools won't work in all versions. My answer is that all the tools I have tested and use regularly do work (power separations, power masking, custom black and white and many others).
Back to the book. It is not a book on what each command does or how to achieve many fancy effects. It teaches you to understand color and tone, analyse an image and use a sound workflow, particularly the use of layers and masking. That is independent of
PSE versions. And it explains why you should do this, and why this method works...
I am now reading and learning Richard's new book about layers. Most of it is really included in the Elements 4 book (except Photoshop specific tools such as blend if or layer groups). That's why, even though the new book is exceptional as a methodical and wonderfully illustrated manual about setting a layer-based workflow, I think you can't go wrong with the PSE4 book.