I argue that the top and bottom textures of the chiseled bookshelf should not rotate depending on the direction the block is facing.

The random rotation of textures like netherrack works because the lighting direction of the texture is ambiguous. For chiseled bookshelves, the lighting originates from the top left of the texture, which is the case for many non-rotatable textures like blocks of gold and diamond.

Regardless of what direction the chiseled bookshelf faces, the top and bottom textures should remain in the "default" north direction.