This is the 1833rd most frequent Korean word.
who
The Korean word "who" (누구) means "what person or which person."
In this sentence, "누가" is used as the subject of the sentence to ask about the person who performed the action (taking the book).
누가 내 책을 가져갔나요?
Who took my book?
Here, "누가" is part of an embedded question to identify the person responsible for the action (doing this work).
이 일을 누가 했는지 본 적이 있나요?
Have you ever seen who did this work?
In this example, "누구" is used as a predicate to ask about the identity of the subject (the person entering the room).