It’s a bread from Brotbackbuch Nr. 1 (Book of Baking Bread, Vol. 1) by Lutz Geissler. The book is German to its core, and I adore it. Lutz emphasizes controlling all parameters of the baking process: weight, time, and temperature. All recipes include the ingredients' temperature, and he gives yeast and salt in hundredths of a gram.

I recall that most of the breads discussed in the book rely on a sourdough starter. Rye-based breads are popular in Germany, and rye really requires a sourdough.

The Rundbrot doesn't have an especially fluffy crumb, and I think it's fair to say that's true for traditional German breads in general. It's a wonderful bread though. Nutritious, with milled and wholegrain flours (wheat, rye, spelt), and a distinct sourdough taste and smell.