Pro tip: had a 30 second talk with a customer that split my whole view on grain direction
Last Tuesday I was at the bindery counter showing a guy his repaired family Bible, and he asked why I lined the new endpapers the way I did. I explained that running the grain parallel to the spine keeps the boards from warping, and he nodded and said his grandfather, a bookbinder in Ohio for 40 years, always ran the grain the other way on small books to get them to open flatter. I have done it my way for 12 years and never had a complaint, but now I am second guessing whether flat opening matters more than warp resistance on books under 8 inches tall. So which side are you on, grain with the spine every time, or grain across it on small formats?
Read somewhere that old hand binders judged grain by how the paper folds, not by spine rules. Small books flex more when you open them so flatness wins over warp. Sounds like grandpa knew his stuff.