This is the 76th most frequent Polish word.
której
"Której" means "of which" or "whose" (feminine singular, genitive or dative case).
'której' is an interrogative pronoun expressing a choice or question about 'drogi' (road).
Nie pamiętam, której drogi wybrać.
I don't remember which road to choose.
'której' functions as a relative pronoun referring to 'dziewczynie' (girl), linking the clause about helping her.
Rozmawialiśmy o dziewczynie, której pomogłeś.
We talked about the girl whom you helped.
'której' is a relative pronoun used here to refer to 'książka' (book), making a connection between the book and its cover.