This is the 932nd most frequent Finnish word.
tuttu
Familiar; acquaintance.
Here, 'tuttu' refers to a person who is known or familiar, emphasizing a previous acquaintance with no strong personal connection.
Hän on vanha tuttu.
He/She is an old acquaintance.
In this case, 'tuttu' describes a place as being familiar, invoking a previous experience or memory.
Tämä paikka tuntuu niin tutulta.
This place feels so familiar.
Here, 'tuttu' is used to express that the subject (the story of the book) is known to the speaker from prior knowledge or experience.