Roll, Slope and Slide - A book about ramps by Michael Dahl is great. I LOVE his books. He has a whole series of books on simple machines and they are all beautiful and clear in their language and illustrations (which he did not do - that was Denis Shea).
The book clearly explains ramps and how they make work easier - and that is job of any simple machine.
Oh, and Dahl lives in a haunted house.