This is an exhaustive and definitive documentation for HTML5 and I really like it. Every element and attribute is listed, defined and there are also examples of them.
The second most used book is the CSS pocket reference, and the first is one I made myself - a notebook where I keep all the niggly things I'm CONSTANTLY having to look up; things like the ascii code for é, and the difference between "display:none" and "visibility:hidden" (a CSS reference). You can't publish a book like that.
The HTML pocket reference goes with me on work trips, because the big version is just too big.
With the proliferation of smartphones, I can't imagine any 'Pocket Reference' being indispensable. This one contains some good information, but nothing you can't find online or in a more comprehensive book.