This is a really common question, but not one that native speakers spend much time thinking about. Look at the following questions:
1) What book are you reading? Or 2) Which book are you reading?
Both questions are grammatically correct; however, there is a slight difference in meaning.
When you have no idea about any details, you will say what. If you have some idea that the person is reading three books now, then you would say, “Which book are you reading now?” Also, when there are a limited number of choices you will use which.