This is the 13054th most frequent Turkish word.
kapağının
"Of its/her/his lid/cover."
In this sentence, 'kapağının' is used to indicate possession, showing that the cover belongs to the book and also specifying something about it, namely its color.
Kitabın kapağının rengini çok beğendim.
I really liked the color of the book's cover.
Here, 'kapağının' demonstrates possession and specificity, referring to the cover of the bag and its zipper.
Çantasının kapağının fermuarı bozulmuş.
The zipper of the bag's cover broke.
In this context, 'kapağının' indicates possession, tying the cover to the box and describing its design.