Answer
In an ideal radiometer, with the vanes in a perfect vacuum, when photons of light hit the reflective side, they bounce and impart more impulse than if they merely are absorbed. Therefore the vanes in the radiometer shown in the textbook would rotate clockwise as viewed from above.
In a consumer-grade radiometer, with air present, this effect is often dominated by another effect. The black side absorbs light and is heated, warming the nearby air in the bulb. The fast-moving air molecules strike the black surfaces harder, resulting in a counterclockwise spin.
