They are not doing work. Students should be able to tell me that rollers reduce the force needed to move the books. The teachers guide for this investigation was printed before an error was discovered. The student observed the number of washers used. The student did not observe the amount of friction.
Machines change the ratio of force and distance.
Some simple machines will merely change the direction a force acts in like a simple pulley. Other simple machines will allow one to exert a smaller force over a greater distance, like an inclined plane. Others will allow one to exert a greater force over a small distance in order to move an object a longer distance, like a broom or fishing pole which are both 3rd class levers. The wheel and axle also take a large force acting on a small distance and produce a smaller force acting over a greater distance. I would not encourage students to conclude that there is less friction or that the rollers are simple machines.