Der Rechenmeister is a historical novel centered on the life and work of Adam Ries, the sixteenth-century German reckoning master whose textbooks helped spread practical arithmetic across Europe. Set against the religious, economic, and intellectual currents of Renaissance Germany, the story follows a gifted teacher and mathematician as he moves through the scholarly and commercial worlds that depend on calculation—mining towns, schools, workshops, and courts. Jörgensen uses the craft of arithmetic as a way into questions of knowledge, reputation, and social change: the new numerals and methods that seem abstract on the page have real consequences for merchants, miners, clerks, and students. The novel presents mathematics not as a detached discipline but as part of everyday life, where skill with numbers can open doors, create rivals, and reshape authority. Written in an accessible narrative style, it combines biographical imagination with a vivid sense of the German past.