It is a book, and at the same time a showcase of what modern browsers can do: curling pages, use keyboard and mouse interfaces to turn pages and go to certain sections in the book, zoom in and out, and change background colors on the fly.
And then for the Tweet that led me to “20 Things”: Google open-sourced the book. The source code is available for anyone to download, and tinker with. It shows the techniques that were used in developing the book (Google App Engine at the back-end, HTML5 at the front-end), and allows developers to learn how to apply the same techniques to their own projects.
All in all worthwhile to check it out, both as a developer interested in the techniques, and a user who is interested in the web.