Modern fantasy depictions of vertical scrolls leave an erroneous impression that the book proceeds in a downward direction, in addition to the cliché use of 'see above' to prefer to anything previously in the text. Hypertext media and text editors further support this misunderstanding by applying continuous scrolling to a document. This confusion is quite new, perhaps as recent as the 1980s.
But, yeah, the horizontal format would've been more common.