The 'which' refers to the subject of the sentence, in this case, 'the series'. So it is saying that the series represents some of Tezuka's most provocative work. You should be able to remove everything that comes after the subject up to and including the word 'which' , and be left with the core of the sentence - in this instance, there is a typo, as it should be represents, not represent. ...... the series represent(s) some of Tezuka’s most provocative work.