This is the 918th most frequent Finnish word.
taakse
"Taakse" means "to the back" or "behind" in English.
In this sentence, 'taakse' is used adverbially to indicate the direction 'behind' in relation to the subject's perspective.
Hän katsoi taakseen nähdäksensä, kuka oli puhunut.
He looked behind to see who had spoken.
Here, 'taakse' forms part of the compound 'taaksepäin', which indicates movement in the direction 'backward' or 'to the rear'.
Autoa peruutettiin hitaasti taaksepäin.
The car was reversed slowly backward.
In this instance, 'taakse' specifies the spatial location 'behind' relative to the object 'shelf' where the book was placed.