But then, another sample sentence here is: 'Αυτό είναι το ρολόι το οποίο αγόρασα. This is the watch that I bought'. For me, 'the watch' is inter-changeable with 'the book' in the two sentences, so I don't see why the pronoun should be changed.
mizinamo, I want to confirm that "που" is exactly the same as "το οποίο" in this sentence. I have also noticed that "Πού" means "Where" at the beginning of a question. The difference is the stress on the "υ". Αm I correct?