A Journey Through Time (German: Eine Reise durch die Zeit) is a book written by H.G. Tannhaus some time betweeen 1953 and 1986. It is a scientific study on time and explores things like time travel, the sun-moon cycle and the nature of wormholes. It is first introduced in "Past and Present" when Helge gives it to Claudia Tiedemann as a gift back in 1986.
It is shown many times during the show and serves the purpose of catalyzying a character's interest in time travel. It's also a narration device, the book's synopsis is the monologue by Tannhaus at the beginning of the first episode, and it also contains theories surrounding time travel that apply to the story.