Paper has grain just like fabric. The highest quality book will be made so that the grain of all sheets of paper is parallel to the spine of the book. Just like fabric hangs more naturally when the grain of the fabric is parallel to the height of your body, the pages lay more naturally when the grain of the paper is parallel to the spine.
To determine the grain of your text block or cover paper, take one sheet and fold in gently in half without creasing the fold. Press down on that rounded edge, without creasing – make a mental note of the strength of the resistance in the fold. Unfold the paper and turn it 90 degrees. Fold the paper in the opposite direction and press down on this rounded fold.
By folding and pressing in both directions you are determining which fold presses down more easily or with less resistance. The grain will be parallel to the fold that presses down more naturally.
Consider what we have just done – we have determined which direction the paper is more willing to be folded and thus have found the grain.