No there probably isn't much advantage of buying the book: 'HTML5 & CSS3 for the Real World' published 2011. Unless you just want to "experiment" and play with making that demo page (HTML5 Herald) they have on the demo mockup site.
In either case some stuff in that book will now be outdated - it's not a reference, or really a tutorial book it just covers some miscellaneous techniques they used to create that demo.
How am I supposed to know it was published in 2011?
I often check Amazon to find out the pub date, if all else fails.
IMHO, I don't think you are wasting your time getting both books. The HTML5/CSS3 one will build on the first by introducing more of the latest CSS/HTML features. The latest Build your Own version gives a brief into to HTML5 and CSS3, but obviously doesn't cover nearly as much as the book dedicated to these topics.
Of course, all of the info is also available online, just not in such a structured way.