Thanks for the suggestions, but it's not exactly a "log" or "journal".
It is also a to-do list (and more).

For example, you would put not only your notes from phone call into it, but also the reminder to place a call. Or if it comes to your mind that you have to buy some cheese when in town, you would write "Get cheese when in town" in it. Maybe it would also contain small calculations, whatever.

This somehow resembles the "ubiquitous capture tool" or even the GTD inbox itself.
And one can easily see the danger of it becoming an enourmous mess (which is exactly what critics say). Accordingly, it is sometimes (but not always) advised to purge older pages every other day, transfer uncompleted items to a new page and cross out the old pages.
This processing step is probably the link to incorporate the book into GTD.
But in the idea of the "super book" usually you're supposed to work from this book.

BTW: "Superbuch" isn't even a proper German word, either, rather a term coined by some time management experts.