The thing with intertextuality is that it is connected up with Reader-Response Theory, which from memory is based on the idea that the meaning is constructed by the reader (so different readers will get different things from the same book) and so in a sense intertextuality encompasses not just the inclusion of texts from other works or the references to them, but also the connotations that those intertexts bring out in the reader's mind. So you get the ideas of shallow and deep reading - shallow reading takes the text as it is, deep reading goes down the rabbit hole of all the intertexts and their connotations/associations, as deep as you like. I hope that make sense.