Amazing Science: Simple Machines (series)  by Michael Dahl and Denise Shea

Roll, Slope, and Slide: A Book About Ramps

Pull, Lift, and Lower: A Book About Pulleys

Scoop, Seesaw, and Raise: A Book About Levers

Tires, Spokes, and Sprockets: A Book About Wheels and Axles 

Looking at Simple Machines (series) by Frost and Helen

          What are Levers?

          What are Screws?

          What are Inclined Planes?