This is the 59th most frequent Lithuanian word.
kurių
"Of which" or "whose" (genitive plural form).
The word 'kurių' here is a relative pronoun in the genitive case, referring to books as a whole group that has not been read.
Yra daug knygų, kurių dar neperskaičiau.
There are many books that I have not read yet.
Here, 'kurių' acts again as a relative pronoun in the genitive case, describing the 'cities' by associating them with their 'architecture'.
Man patinka miestai, kurių architektūra yra įspūdinga.
I like cities whose architecture is impressive.
In this sentence, 'kurių' acts as a relative pronoun in the genitive case, linking the 'questions' to their 'answers'.