An orihon is the step between a
scroll and the modern book -- it is a
book formed by
accordion-
folding
paper,
parchment, or
vellum and writing or printing on one side of the material so that one column of text appears between each set of folds. (This is also called a
concertina book, after a musical instrument related to the accordion.) Sometimes a binding would be attached by sewing one side of the pages together (making it look like the Western
codex book, but with what would be one sheet of paper in a codex really two thicknesses instead). Just as often, though, it was left unbound and
covers were just attached to the ends of the paper for protection when it was not being read. This
format was easier to use than a scroll, though not so convenient for large amounts of
information as the codex would later turn out to be.
The name comes from the Japanese: "ori" (fold), + "hon" (book). The Chinese also used this format; it was popular for Buddhist works from perhaps as early as the eighth century until the fourteenth century when the codex format took over. "The cascading movement of the pages as they were passed from hand to hand was said to create healing properties." However, it was also sometimes seen in the West before the codex style took over.
Sources:
http://wordsmith.org/awad/archives/0402
http://palimpsest.stanford.edu/don/dt/dt2398.html
http://www.cbbag.ca/TheBook.html
http://www.swt.edu/~pg48478/GuitronUnitAccordionBoundlesson.htm
http://www.hotlinecy.com/Granger/anatomy%20of%20book.htm
http://62.1911encyclopedia.org/B/BO/BOOKBINDING.htm