Shelf Grid Alignment

I climbed onto the lowest shelf of the bookcase to inspect the vertical book arrangement. Five paperbacks stood in a row, but the third volume tilted sideways at approximately fifteen degrees, interrupting the perpendicular line. I stepped forward along the shelf's straight wooden edge and leaned my shoulder against the spine of the leaning book. With sustained pressure, I pushed it upright until it met the adjacent volume at a clean ninety-degree angle. The row forms an uninterrupted plane once more.