A Journey Through Time (German: Eine Reise durch die Zeit) is a book written by H.G. Tannhaus some time betweeen 1953 and 1986, published by Mino Taurus. 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 "Lies" when the stranger comes into the room after the shower and looks at the book on which lies a strange key.
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.
On the back cover of the book, a following blurb is written:
"Wir vertrauen auf den linearen, ewig gleichförmigen Verlauf der Zeit - bis in die Unendlichkeit. Aber die Unterscheidung zwischen Vergangenheit, Gegenwart und Zukunft ist nichts als eine Illusion."
(English translation: "We trust in the linear, forever the same shape of the past, until eternity. But the diffrences between the past, presence and future are nothing but an illusion.")