This is the 2438th most frequent Dutch word.
welk
"Which."
Here, 'welk' is used as an interrogative adjective referring to a specific book.
Welk boek lees jij?
Which book are you reading?
In this instance, 'welk' modifies the noun 'huis' in a subordinate clause.
Ik weet niet naar welk huis ze gaan.
I don’t know which house they are going to.
'Welk' is used here as an interrogative pronoun in an indirect question.