The world that is a book is devoured by a reader who is a letter in the world's text; thus a circular metaphor is created for the endlessness of reading. We are what we read. The process by which the circle is completed is not, Whitman argued, merely an intellectual one; we read intellectually on a superficial level, grasping certain meanings and conscious of certain facts, but at the same time, invisibly, unconsciously, text and reader become intertwined, creating new levels of meaning, so that every time we cause the text to yield something by ingesting it, simultaneously something else is born beneath it that we haven't yet grasped. That is why - as Whitman believed, rewriting and re-editing his poems over and over again - no reading can ever be definitive.
Alberto Manguel, from A History of Reading