This is the 175th most frequent Swedish word.
vilka
"Vilka" means "which" or "who" (plural).
In this question, 'vilka' is used as an interrogative pronoun referring to plural objects (books).
Vilka böcker ska vi läsa till lektionen?
Which books should we read for the lesson?
In this exclamation, 'vilka' emphasizes the admiration for the plural objects (paintings).
Vilka fantastiska målningar han har skapat!
What fantastic paintings he has created!
Here, 'vilka' is used as a relative pronoun introducing a dependent clause that refers to an unknown plural subject.