By the fact of being bound, books have an order. They advance from beginning to end as pages are flipped. Pages of text are also ordered, as lines and columns in sequence. In most books, these orders mirror the order of content, so that a biography or a novel, say, can be read in its intended order by simply “following” the book. Works of reference, by contrast, were printed as books and could, in extreme cases, be read cover to cover, but they were designed to allow multiple points of entry and exit.