Das Buch, as it was called at the table, is a collection of scenarios for Catan. There are fifteen different rule sets in the book that tweak Catan, as well as the components in order to implement them. It’s basically a huge set of expansions. I’m also pretty sure that it was only ever published in German, although translations were available. (I think Mayfair provided one and I think that’s the one we used)
Sadly, I don’t remember Das Buch that well. It was a couple different lifetimes ago. I know that I played three different scenarios but the Great Race one is the only one I remember. Which I might have won and definitely thought was fun.
Now that I remember that Das Buch actually exists, I really want to go and take a look at it. It feels like it could be great or possibly just great for its day. Some games, like Memoir 44, thrive on having variants and scenarios. And Sid Sackson knows, Catan has had a lot of them, although I don’t know if I’d say all of them have thrived for me.
I know there are English translations floating around the internet. Boardgame Geek itself has a couple. So I can at least read the scenarios, even if actually playing them might be tricky. Could be amazing discoveries. Could be just interesting historical footnotes.