When would "ποιο", "ποια," and "τι" be used as the interrogative pronoun "what"? Would one be used specifically for something?
One explanation I have read is " the difference in meaning between ποια βιβλία and τι βιβλία is that the former normally implies that there is a set of books already known to both speaker and hearer, and the speaker is asking the hearer which books out of this set s/he is referring to; while in the latter the speaker implies " which books out of all the books that exist".".
Compare, by the way, with cuál vs. qué in Spanish—essentially the same difference.