Paper in books is also aligned the same way, to resist better to humidity. Humidity enlarges the fibres, but doesn't get them longer. In a humid environment a

Paper in books is also aligned the same way, to resist better to humidity. Humidity enlarges the fibres, but doesn't get them longer. In a humid environment a book grows wider but not taller. If the paper is oriented the other way, the paper would grow taller and the spine of the book, which is stitched and glued, would force the paper to warp and became ondulated.